Update servo arm only on real change.

Accelerometer calibration bugfix

6point accel calibration

Still threshold increased to 0.1m/s^2, and orientation error threshold to 5m/s^2. Timeout increased to 30s.

mkblctrl: some small fixes

Help Parameter added and some small fixes. This Version was flown several Hours without any Problems.

don't update the voltage/current values on error

this integrates the current over time to calculate discharged_mah, and
allows the scaling of the current and the bias to be set with the
px4io command

the timeout of 1ms was far too long, and could impact flight
performance

Returning 0xffff on error matches the FMU code, and allows bad values
to be discarded

we don't know how to scale it as we have no info on what sensor is
attached. As we are returning a uint16_t it is better to let the FMU
sort it out or we'll just lose precision.

this allows setting of the channel mask directly, which is useful for
testing

this should release it for PWM use

let the IO board decide if the rate is reasonable, and limit it there

this fixes the rates on ArduCopter, which try for 490

otherwise we print thousands of 00000 characters

when windows reconnects to a ACM device, this assert sometimes
triggered. The case it is looking for is handled further down.

this allows for flags needed for the APM build