mpu6000: added factory self-test function

this follows the factory calibration self-test method in the datasheet
to see if the sensor still has the same calibration it had in the factory
mpu6000: monitor some more registers
mpu6000: added "mpu6000 testerror" command

used to generate a error case for reset testing
mpu6000: wait for 10ms after a full reset

this prevents the mpu6000 getting in a really weird state!