System always contains root dir.

In recovery, system image is mounted to /mnt/system. After the work of
first stage ramdisk, system image always contains the root dir, no
matter what value of ro.build.system_root_image is.

Test: do compat check in recovery
Bug: 112586588
Change-Id: I3890c9c1335b23fd22a8910bca07e59d631bac71
1 file changed