build: Fix "adb shell" in recovery for system_root_image.

For system images that contain the root directory, "adb shell" in
recovery stops working if the image is mounted at "/system", because
sh now locates at /system/system/bin/sh. We fix the issue by mounting
the image on /system_root and create a symlink to /system_root/system
for /system.

Bug: 22855115
Change-Id: Ia6b257b75aeb67687c3909063d1725a23a5ccd60