g3: add bump script

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

Change-Id: Ib04dda56564c8b89f9c24a75ef5dffc43073b063
g3: enable BOARD_CUSTOM_BOOTIMG

Change-Id: Ifc6bb57911fc65d579b81f4e97672426c07948f7
g3: disable some overlays

Change-Id: I73c796f8136d1ee942e680f101b61e570045a814