bootcode: Fix for B+ waking spuriously after shutdown
See: http://www.raspberrypi.org/forums/viewtopic.php?f=63&t=82188#p581014

firmware: image_encode: reset line count at end of image
See: http://www.raspberrypi.org/forums/viewtopic.php?f=43&t=80728#p576146

firmware: gencmd: Add command to read the ring oscillator
See: http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=82369

firmware: arm_loader: Add displayed warning for over-temp and under-voltage

Also limit overclock on under-voltage (optionally disableable)
Use avoid_warnings=1 to disable warnings