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.

Bug: 110097226
Change-Id: Ibd011c6f2a7cdfdc36e695e717b337b563784876
3 files changed