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
