g3: add bump script

* auto bumps boot and recovery images
* to enable for your variant, set TARGET_REQUIRES_BUMP in BoardConfig

Change-Id: Ib04dda56564c8b89f9c24a75ef5dffc43073b063
bump: clean up the script

Change-Id: Idd72403f7f01565a7acd2f8a2a5b89cfd2016618
(cherry picked from commit 1ccb3f773c543fc39d9ae4a679d43ba79cf4d658)
Merge remote-tracking branch 'upstream/cm-12.0' into lollipop