build: fix BOARD_CUSTOM_BOOTIMG_MK

Change-Id: I1b9fa7d8eb67bdcaaae7b55ca538b33545e73581
build: add support for custom releasetools

Change-Id: I2337559ac669bb5868708c4726cbacbf5713261a
Use prebuilt boot image

Change-Id: Ic348d31e8df7933dc4f9f055b7a5a2beacc21d32