firmware: MJPEG Encoder: Block when insufficient output buffer space
See: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=43&t=62364&start=150#p481779

hello_pi: Some minor fixes.
See: https://github.com/raspberrypi/firmware/issues/240
See: https://github.com/raspberrypi/firmware/issues/241