app: aboot: update commandline for system as root behaviour.

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

This change ensure we are updating commandline with skip initram fs
to skip mounting of ramfs from boot image, and append init path and
system partition block.

Change-Id: I866cafedf3efb481b5eca2d331dc94a2e6314381