Recent devices (e.g. Pixel 2XL aka taimen) do not define any "system"
partition in their fstab, so a plain "mount /system" fails.
Furthermore, trying to get the entry from /etc/recovery.fstab also fails
because the partitions have slot suffixes (e.g. system_a).
So for these devices, we need to figure out the active slot, mount the
system partition (which really contains the whole root) and lastly mount
the actual system partition from the rootfs.
Change-Id: Ibb73a82896c1f6ce6af9c334b8d0908a183913b9
