Enable BOARD_BUILD_SYSTEM_ROOT_IMAGE.

This enables a mode where the initramfs (ramdisk.img) and system.img
files are combined into a bootable 'root' system.img. It is needed for
GSI compatibility and treble compliance.

The device tree blob is also moved over to the 'cuttlefish' project as
it can no longer be shared with previous releases (because 'system'
cannot be defined in the fdt any more.)

Bug: 110097226
Change-Id: Ibd011c6f2a7cdfdc36e695e717b337b563784876