fs_mgr: overlay: device tree and system as root

fs_mgr_overlayfs_mount_all() should pick up /system even if device tree
does not specify / or /system.

Because of change where we pass fstab to fs_mgr_overlayfs_mount_all(),
/ (or rather /system) overlayfs mount got shifted to early init and
did not occur in first stage init. We need the assumption that /
mount is implied when not specified in the dt fstab to be considered
by overlayfs logic.

Test: manual
Bug: 109821005
Bug: 115751838
Change-Id: Ia8fc5d8e8cc50c66197a91efa80e46d66c2d108d