build: Only mount /system if not mounted

The commit on which this depends takes care of the mount error if /data
is already mounted during install. This commit takes care of the error
if /system is already mounted.

Change-Id: I97d0540676a7ce98762545e76ecb1a7f6335d204
build: Only mount /system if not mounted

The commit on which this depends takes care of the mount error if /data
is already mounted during install. This commit takes care of the error
if /system is already mounted.

Change-Id: I97d0540676a7ce98762545e76ecb1a7f6335d204
fix build for unified devices

Change-Id: I003301c1560358056b4e0e4c8fbfc16a1f0e74d8
Signed-off-by: cj360 <ayunker551@gmail.com>
Merge branch 'lollipop' of https://github.com/AOSPA-legacy/android_build into HEAD