hammerhead: set tags_offset for BOARD_MKBOOTIMG_ARGS
hammerhead: set tags_offset for BOARD_MKBOOTIMG_ARGS

Change-Id: I264dd67c4cb23170524fa640e7e531f4a543935b

maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead