CameraStab: send PIOS_CAN_GIMBAL message on update

This passes to the gimbal the desired attitude (to integer
angles) and the attitude of the flight controller. This
will allow more efficiently controlling the attitude with
POI mode or relaying the setpoint from the transmitter.
UAVORelay: hacky send and receive custom messages
UAVORelay: receive CAN messages for gimbal
Targets: UAVORelay only useful for SparkyBGC

Make it always on for this board
UAVORelay: rename to GimbalControl

This code was only serving one purpose, so this makes it
more explicitly.
PiOS CAN: clean up support for multiple IRQ

Current code hardcodes CAN1 everywhere. This abstracts
that to a single place.
BrushlessGimbalSettings: increase max default rate

Having a rate limit by default just causes it to slip
at high rotation rates. Not needed.
Sparky/SparkyBGC: change CAN rate for F4 compatibility

To make sure that F3 and F4 boards can communicate the
prescalars are set to the APB1 rate of the board divided
by 2. This makes the baud rate independent of clock speed.