AP_Param: explicitly print floats to 6 dec places

C++ default is to print 6 decimal places but nuttx displays none by
default
DataFlash: explicitly print floats to 6 dec places

C++ default is to print 6 decimal places but nuttx displays none by
default
Iris: Change Flight Modes
Plane: fixed uartA baud rates for APM1
Rover: fixed baud rates on APM1
Copter: fixed baud rates on APM1
Plane: only send RC_CHANNELS_SCALED in HIL

saves a bit of telemetry bandwidth
Copter: only send RC_CHANNELS_SCALED in HIL

saves a bit of telemetry bandwidth
Rover: only send RC_CHANNELS_SCALED in HIL

saves a bit of telemetry bandwidth
Plane: run SENSOR_OFFSETS message at 1/10 of the requested rate

this message wastes bandwidth, as the values change very slowly
HAL_AVR: changed to C implementation of float formatting

Many thanks to Soren Kuula for this work!
HAL: added Printf test sketch

for testing new non-asm printf code
AP_HAL: moved printf implementation from AP_HAL_AVR to AP_HAL

this gives a common printf implementation on all boards, which
simplifies testing, and also simplifies porting to new boards
AP_HAL_AVR_SITL: convert to new AP_HAL printf
AP_HAL_Empty: convert to new AP_HAL printf
AP_HAL_PX4: convert to new AP_HAL printf
AP_HAL_SMACCM: convert to new AP_HAL printf
AP_HAL: expanded printf test suite
AP_HAL_AVR: removed unused script
AP_HAL_SMACCM: use common printf implementation