Copter: remove unused variable from Single copter
CopterMotors: fix example sketch
AP_InertialSensor: added INS_PRODUCT_ID values
AP_InertialSensor: use fabsf()
Copter: pre-arm check of INS health
Copter: ARMING_CHECK made into bitmask

Allows arming checks to be individually enabled or disabled for baro,
compass, GPS, INS, parameters, RC and board voltage