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

Set in board file with TARGET_OTA_ASSERT_DEVICE.

Conflicts:
	tools/releasetools/ota_from_target_files

Change-Id: I27f89f440bdc4982dcac35631496c865f2657c83
Fix build on asserts

Change-Id: Ia28b014ac029a8c8011ab5535aeaea215199c01f