Beta mavlink2: onboard camera capture 

This adds the mavlink mode CAMERA which sends the following to an onboard camera module connected to the FMU:

- heartbeat
- attitude
- global position
- camera capture command on if armed and off if disarmed

start the mode on Telem2 (on Pixhawk):

       mavlink start -d /dev/ttyS2 -b 57600 -m camera