Create a Falcon mode updater image

[alchark]

Build a per-board image containing boost+TPL+SPL+Linux+initramfs suitable for direct booting on RK3576 to facilitate on-device upgrades (recovery-like).

• RKNS packed image suitable for loading from local storage (e.g. from an emergency SD card)
• Loader (i.e. Maskrom) packed image (usb471+usb472) suitable for booting directly from RAM via USB upload

Depends on #123