Support Raspberry Pi USB boot

[clach04]

Enhancement request. Lakka 2.2 (possibly earlier) does support booting from USB with Raspberry Pi 3b+ (and some earlier models via https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md) if cmdline.txt is modified (by default it is hard coded to the SD card device/partitions).

There are notes online on how to do this, e.g. https://forums.libretro.com/t/raspberry-pi-3-usb-boot/12172/13 I believe this can be supported out of box without the need to edit the cmdline.txt manually.

I was able to get a USB boot with Lakka 2.2 by using the LABEL option for both "boot" and "disk".

Original:

boot=/dev/mmcblk0p1 disk=/dev/mmcblk0p2 quiet $EXTRA_CMDLINE vt.global_cursor_default=0 loglevel=2

Boot using label:

boot=LABEL=LAKKA disk=LABEL=LAKKA_DISK quiet  vt.global_cursor_default=0 loglevel=2

It looks like

Lakka-LibreELEC/scripts/mkimage

Line 175 in 8c3a7e6

boot=/dev/mmcblk0p1 disk=/dev/mmcblk0p2 quiet $EXTRA_CMDLINE vt.global_cursor_default=0 loglevel=2

is the one to change (I don't have a build environment :( ).

[clach04]

I opened PR #612 however I checked up stream and https://github.com/LibreELEC/LibreELEC.tv/blob/e370750526f8196252aadfab97f065e37e67e172/scripts/mkimage#L182 already uses UUID its already a resolved issue!. Better to pull upstream and abandon #612.