edify_generator: Mount: Only mount if a partition is not already mounted

Change-Id: I7f41811fabfa4e6dd90a8d56cd1036bcf3066fed
Make sure that /system is appropriately mounted before starting installation

Change-Id: If8470d973d69e94a406ea032321288c2a66943ee
Merge remote-tracking branch 'upstream/L5' into L5