Merge "DO NOT MERGE: Add the option to reserve headroom for partition images....

Merge "DO NOT MERGE: Add the option to reserve headroom for partition images. This is useful for devices with low disk space with different build variants." into cw-f-dev am: 1590301a
am: 92aa9636  -s ours

Change-Id: I1dbb5a705f4aca21864c1072bd39f7924d43a4f8