Whole OS image replacement with TUF

[jkaczman]

Integrate https://theupdateframework.io into the update workflow to allow for entire OS image replacement.

• Simple update client acting as an intermediary between TUF and the bootloader backend - A/B update client using The Update Framework (TUF) core#126
• Find suitable bootloader backend (derived via RAUC PR) - https://github.com/Rtone/raspberrypi-firmware-rauc-bootloader-backend
• /etc/rauc/system.conf, example:

#	(...)
# 	[slot.boot.0]
# 	device=/dev/mmcblk0p2
# 	parent=A
#	(...)
#
# 	[slot.boot.1]
# 	device=/dev/mmcblk0p3
# 	parent=B
#	(...)
#
# 	[slot.rootfs.0]
# 	device=/dev/mmcblk0p5
# 	bootname=A
#	(...)
#
# 	[slot.rootfs.1]
# 	device=/dev/mmcblk0p6
# 	bootname=B
#	(...)

• Modify OS image to use the update client with a timer service.
• Modify OS image partitions use a slot system.
• Helper CLI to coordinate TUF additions (use R2 API, work with Yubikey, stage changes)

[jkaczman]

Links into secluso/mobile_client#40