this also brings GCS_Mavlink.pde closer to the plane implementation

support 3 UARTs on non-AVR platforms

PX4 and SITL get an extra UART

TradHeli: Fix for yaw offset switching.  Yaw Offset should switch on and off with the main motor, not waiting until the rotor has reached full speed.

Loiter's Roll-Pitch mode was not being set on start-up.

we know that packed data structures are not as efficient

this allows for secondary sensors

We now limit the target stopping point to 1x the xy leash length while
previously it was 2x.  This is justified because this limit is only used
when the copter is travelling at higher speeds but at higher speeds air
drag tends to make the copter stop more quickly naturally.

Without this initialisation the first RTL could be too aggressive as it
tries to stop too suddenly

The main rotor ramp was being held back by the rotor speed estimate
instead of being allowed to jump up to the estimate if it's lower.

Also fixed some incorrect indentation

Set FRAME parameter to 10