module to use radio channel as remote camera trigger

	new file:   conf/modules/digital_cam_shoot_rc.xml
	new file:   sw/airborne/modules/digital_cam/dc_shoot_rc.c
	new file:   sw/airborne/modules/digital_cam/dc_shoot_rc.h
not really needed, as it will fail to compile anyway if `DC_RADIO_SHOOT` is not defined.
Would make more sense to print a useful error message if it is not defined.