Add usart 6 to px4-stm32f4discovery board

set default, weak rate I gains for multirotors

make main LED show low and critical battery status

mpc parameter INAV_W_Z_VIS_P increased

Fix potential null pointer deref in Mavlink dtor if task_main returns error

LL_APPEND is called just before the loop spins up but various error conditions can cause the task to exit before then.
When that happens Mavlink::start_helper calls delete on the instance which tries to prune it from the global list.
If this is the first Mavlink instance to attempt starting the list head is null and we hardfault in the Mavlink dtor.

Only call LL_DELETE after checking the list head for a null pointer.

added default rate I gain for multicopters. added default gains for FireFly6 in mc mode (from Simon Wilks)

mc_pos_control: always update previous velocity

commander gyro calibration: Do not require a specific position, automatically start a retry after motion on the first try

port #1920 to mavlink_receiver
fixes #1921

copy quaternion setpoint into attitude setpoint topic

this runs the mpu6000 200usec faster than requested then detects and
disccards duplicates by comparing accel values. This avoids a nasty
aliasing issue due to clock drift between the stm32 and mpu6000

this runs the sampling of the accelerometer 200usec faster than
requested and then throw away duplicates using the accelerometer
status register data ready bit. This avoids aliasing due to drift in
the stm32 clock compared to the lsm303d clock