this allows us to put a serial port into non-blocking mode, so that
writes that don't fit in the transmit buffer are dropped. This will be
used in flight to prevent stray printf() calls from causing large time
delays in the code

the 128 byte serial transmit buffer was causing significant problems
with queueing of mavlink messages. With 256 bytes we can fit a lot
more messages out in each pass of the code, which makes telemetry more
efficient

As we discussed on the dev call, we now have enough free ram for this
to be worthwhile

we now have the EEPROM option COMPASS_AUTODEC instead

when this is 1 (which is the default), we will get the declination
automatically via the AP_Declination library

when it is 0 we will use the value configured by the user

there was a comment saying this was too slow, but it actually costs
about 30 usec extra, which is trivial given the full auto declination
call costs 680 usec and its only called once per boot

AP_Declination: Update LUT based on the changes that I made in the LUT generation code. This LUT is the final version that has no truncation.

ArduCopter: Changed implementation of configuration value for automatic declination. There is now a FORCE_AUTOMATIC_DECLINATION_UPDATE that when enabled will update the declination on every GPS 3D fix regardless of whether or not the user saved a value to the EEPROM. By default the declination will only be set by the automatic declination routine if the user has not saved a declination to the EEPROM.

AP_Declination: Removed trailing zero deltas as they have no impact on the final value and only take up extra bytes.

AP_Declination: Updated compressed lookup value table. Located 9 more failure points due to truncation on the packed LUT.

AP_Declination: Found 7 of the 22 failure points and fixed them. The issue with those 7 was the way I originally packed the value array. The last delta values on some rows were getting truncated. The new number of failures is down to 15 - WIP.

AP_Declination: Updated test to run in 5 degree increments. Changed to only print failures and also print total pass vs total fail.

AP_Declination: The exception signs unpacking logic was incorrect. I was shifting a 1 value left y%8 which would have needed a the signs to be packed right to left. My packed signs byte was packed left to right (left most being the 0 position) so I needed to reverse the shifting.

AP_Declination: In order for the bitwise & operation to work, negative signs need to be represented by 1s instead of 0s. Changed so that negative signs are represented by a 1 in the packed signs array. Updated lookup logic to reflect this change.

AP_Declination: Added original array to test sketch. Added loop to validate new compressed array against original in 1 degree steps.

AP_Declination: Moved rows with large spikes in delta to their own unsigned exception rows. The signs are in a separate packed array.

AP_Declination: Implemented Delta Encoding and Run-Length Encoding. Added method to traverse the compressed array and return the lookup value based on the same lat_index/lon_index that was used before.

AP_Declination: Reduced the size of the lookup table to support -75 <-> 70 latitude range. This allows the datatype to remain int8_t.

fix possible issue loading stream rates
fix tracker ranges
add more ch6 options
modify mag calib with throttle

when MANUAL_LEVEL is set to 1, we don't do accelerometer levelling at
startup, and instead used the values saved in the EEPROM. This makes
it easier to do levelling on the bench, or once for a series of
flights for the day

when a user first connects with USB, and later switches to the
telemetry port without restarting we were getting zero for error_yaw
in the logs, as AHRS.get_error_yaw() was being called twice.

This ensures we give the last value after the counter is reset

explain the algorithm a bit more

        #define CH6_YAW_KI 24
        #define CH6_ACRO_KP 25
        #define CH6_YAW_RATE_KD 26
        #define CH6_LOITER_KI 27
        #define CH6_LOITER_RATE_KI 28