[automerger skipped] Add boot.img substitution to the build_mixed script.
am: aabbd91ab5 -s ours
am skip reason: change_id I2877e79522b5836e876c87f38221ca6890c41ce6 with SHA1 22da1576b1 is in history

Change-Id: I2678fcdfb580183c71858775c8cc7c17907c435d