Allow override of device asserts, including multi-device support.

Set in board file with TARGET_OTA_ASSERT_DEVICE.
(cherry-picked from commit 0f452f21fc9323b9d1fe746161761cf40aaa5030)

Change-Id: Icd10de924f2b88168912c9575174cebf6b0104ba

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files