recovery: Allow "Mount /system" for system_root_image.

When system images contain the root directory, there is no entry of
"/system" in the fstab. Change it to look for "/" instead if
ro.build.system_root_image is true. We actually mount the partition
to /system_root instead, and create a symlink to /system_root/system
for /system. This allows "adb shell" to work properly.

Bug: 22855115
Change-Id: Ibac493a5a9320c98ee3b60bd2cc635b925f5454a