Custom boot image/recovery image capabilities credit CM

Summary:
We need this for G2, and likely other devices going forward. Goes
with the CM mkbootimg source commit in system/core

Test Plan: confirm functionality on custom and regular boot images

Reviewers: synergy

Differential Revision: http://review.codefirex.com/D60

Conflicts:
	core/Makefile

Conflicts:
	core/Makefile
	tools/releasetools/common.py