The compass.configured() method checks the eeprom and cannot be const
meaning the Arming object requires a non-const reference to the compass.
Removed check for null compass because the compass object is always
created although this could lead to unnecessary pre-arn check failures
even though compass-use is set to false.
This checks that when compass offset learning is off, that the offsets
are non-zero and that the detected device id matches the device id
stored to eeprom when the offsets were last saved.
Randy Mackay
Andrew Tridgell
Don Gagne
Michael Day