target: init: Add support for system-As-Root on non-A/B for VB1/VB2 targets.

According to System-As-Root behavior, system.img should contain both
system content and ramdisk content, and should be mounted at root(a/b).

This change adds api to check if the target uses system as root build.

Change-Id: Ieb52de6ac5e464c94128e322b9d92e46114abf03
2 files changed