SPL and UCL-compressed bootloader are now packed into one output,
bootloader.m3k, eliminating the separate SPL build phase.
The Rockbox bootloader now has a recovery menu, accessible by
holding VOL+ when booting, that lets you back up, restore, and
update the bootloader from the device.
Change-Id: I642c6e5fb83587a013ab2fbfd1adab439561ced2
