AP_Compass: fixed HMC5883's initialisation code
AP_HAL: added Zynq platform type
HAL_Linux: Basic Zynq Linux platform support

Preliminary support for Zynq/Linux on the 'ZyboPilot' HW platform.

see https://github.com/trjw/ZyboPilot-bsp for the PetaLinux / Vivado project
files.

At this stage CPPM/Pulse in and PWM out work, and in CLI mode the passthru
test works correctly.

The platform has passed minimal smoke testing in HIL mode.

ZYNQ IS NOT FLIGHT TESTED YET!  FLY THIS PLATFORM AT YOUR OWN RISK!

Signed-off-by: John Williams <john@whelanwilliams.net>
Build: added zynq make target