Mark custom properties in image schema as non-base

Currently it is impossible to determine if given image schema property
is base or custom one and knowledge of that can be handy in some
situations.    Proposed change appends to every custom property special
key which determiness that it is not a base property.

Change-Id: I49255255df311036d516768afc55475c1f9aad47
Partial-Bug: #1371559
(cherry picked from commit 94c05cbdbb3a78b3df4df8d522555f34d2f0a166)
Merge "Mark custom properties in image schema as non-base" into proposed/juno