PX4: Changed the startup script to have mavlink on USART2 / Serial 1 on FMUv2
HAL_PX4: support up to 16 analog channels
PX4: allow NuttX tree to be in directory above Firmware tree
Copter: ask for IMU samples at 100Hz

this produces the same result, but makes things simpler in the driver
PX4: don't need hil module
AP_InertialSensor: run PX4 FMUv2 at 800Hz

this gives much better noise suppression