Update tooling for custom kernels (#6)

Improve support for custom device trees when kernel updates are received

* Modified make-trees.sh so that it builds for all installed kernels.
* A new script flash-latest.sh that copies the resulting .dtb files into the /etc/flash-kernel/dtbs/ override folder.

The build-trees README is updated for this, and shows how to configure flash-kernel to use the custom .dtbs.

Kernel upgrade should now go:

[after a new kernel upgrade is installed]
* Upgrade source: cd source && apt source linux-riscv
* Build dtb's: cd build-trees &&; ./make_dtbs.sh
* Install device tree via softlinks ./flash_latest.sh
* Reboot

GPIO-examples.md

@@ -0,0 +1,125 @@
+# GPIO usage examples (for the MQ-Pro)
+This guide assumes you have a correctly installed and set up board, with the correct device tree (plus overlays) to expose the pins you want to use.
+
+*Caveat:* notes here are biased towards Python usage, since that is what I will be using in my projects.
+
+## General Purpose GPIO (digital read/write)
+**You do not need to use a custom Device Tree in order to use digital IO**
+* The 'default' device tree for the MQ pro has 26 free pins to use! 
+
+Look at the great guide here: https://worldbeyondlinux.be/posts/gpio-on-the-mango-pi/
+
+It does a better job of explaining this than I can do in a short guide.
+
+## PWM
+**working**, There are eight PWM timers available and GPIO pins can be mapped to these in a custom device tree
+- The available mappings are somewhat limited, see the diagram in the main README to determine which pins on the GPIO connector can be used.
+- The example below uses (legacy) `/sys/class` control, which in turn needs root access. PWM control from userland seems like a WIP for linux at present.
+- I have not (yet) investigated using this via `lgpio` in Python.
+
+The following needs to be run as root. It uses `pwm2` (the `lora` example device tree attaches this to pin 31 on the GPIO connector).
+
+First, export the PWM interface:
+```
+# echo 2 > /sys/class/pwm/pwmchip0/export
+```
+- The node for the interface wil appear at `/sys/class/pwm/pwmchip0/pwm2/`
+
+Set a default period (10μs) and duty cycle (5μs, 50%):
+```
+# echo 10000 > /sys/class/pwm/pwmchip0/pwm2/period`
+# echo 5000 > /sys/class/pwm/pwmchip0/pwm2/duty_cycle`
+```
+After setting the default you can enable it with:
+```
+# echo 1 > /sys/class/pwm/pwmchip0/pwm2/enable`
+```
+You can stop and detach the interface with: `# echo 2 > /sys/class/pwm/pwmchip0/unexport`
+
+The following is a shell script that implements a crude LED fader:
+
+```bash
+#!/bin/bash
+# PWM silly fader
+#
+pwm="/sys/class/pwm/pwmchip0/pwm2"
+
+echo normal > $pwm/polarity
+echo 10000 > $pwm/period
+echo 1 > $pwm/enable
+while true ; do
+    for p in 40 100 400 1000 4000 10000 6000 1200 600 120 60 0 ; do
+        echo -n "."
+        echo $p > $pwm/duty_cycle
+        sleep 0.25
+    done
+    echo
+done
+```
+See the [kernel guide](https://www.kernel.org/doc/html/latest/driver-api/pwm.html#using-pwms-with-the-sysfs-interface) for the parameters we set to assign and control the pin.
+
+## I2C
+**Working**: I have read temperature, pressure and humidity from a BME280 sensor connected to pins `3` and `5`.
+
+Install [`pypi:bme280`](https://pypi.org/project/bme280/) and it's requirement `smbus-cffi`.
+* I am using a [virtual environment](https://docs.python.org/3/tutorial/venv.html), rather than installing globally.
+* Add the user to the group 'i2c' and re-login.
+```
+$ sudo apt install python3-venv python3-dev i2c-tools
+
+$ sudo usermod -a -G i2c <username>
+# Log out then in again so that your user now has the `i2c` group membership
+
+# Create virtualenv in a directory './bme-env' and activate it (exit with `deactivate`, removing the directory+contents deletes the venv)
+$ python3 -m venv bme-env
+$ source bme-env/bin/activate
+(bme-env) $ pip install --upgrade pip
+(bme-env) $ pip install --upgrade smbus-cffi bme280
+
+# The bme280 library provides a python API, and a commandline tool
+(bme-env) $ which read_bme280
+<cwd>/env/bin/read_bme280
+
+# My bme280 defaults to address 0x76, and I'm using I2C0
+(bme-env) $ read_bme280 --i2c-bus 0 --i2c-address 0x76
+1024.85 hPa
+  56.84 %
+  21.59 C
+```
+
+## SPI
+**Working?**: When I enable SPI1 in the device tree nodes a device at is registered at /sys/devices/platform/soc/4026000.spi/, it lists it's driver (correctly) as sun6i-spi and is a bus master.
+* Kernel drivers that use spi via pinctl should be able to use this.
+* But no block device appears at /dev/spi*.
+  * Normally spi-tools provides userland support via the /dev/spi* device.
+* I do not plan to use SPI so I have not tested further.
+
+---------------------------------------------------------
+
+# Extra!
+## Status LED:
+The onboard (blue) status LED is attached to gpio `PD18`, and can be controlled via the sys tree:
+
+`$ sudo sh -c "echo 1 > /sys/devices/platform/leds/leds/blue\:status/brightness"` to turn on
+
+`$ sudo sh -c "echo 0 > /sys/devices/platform/leds/leds/blue\:status/brightness"` to turn off
+
+You can make it flash as network traffic is seen with:
+
+`$ sudo sh -c "echo phy0rx > /sys/devices/platform/leds/leds/blue\:status/trigger"`
+
+Other control options are available, `$ sudo cat /sys/devices/platform/leds/leds/blue\:status/trigger` shows a list and the current selection. Most do not work or are not very useful; ymmv.
+- `PD18` can also be re-mapped to `pwm-2` in a modified device tree if you want to manually control the LED and vary it's brightness.
+- PD18 is also used as the LED_PWM pin on the DSI/LVDS output
+
+
+## Onboard CPU temperature sensor:
+```console
+$ sudo apt install lm-sensors
+$ sensors
+cpu_thermal-virtual-0
+Adapter: Virtual device
+temp1:        +19.4°C
+```
+**HOWEVER**: This is nonsense.. I'm testing with the board in a enclosure, and the attached BME280 sensor is showing room temp outside the enclosure as 22C, the CPU is not that cold.
+- check out the device tree, maybe a bad offset. Or some kind of calibration/reference voltage needed?