add ability to remove device assertions

do this by setting adding "TARGET_OTA_ASSERT_SKIP := true" in BoardConfig.mk

Change-Id: Ia832748ddc00320e0193d93be0883f4cc9208a35

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files

Conflicts:

	core/Makefile
	tools/releasetools/ota_from_target_files