Cleanup sensor auto detection code.
Decoupled mpu6050 acc and gyro detection code.

This commit cleans up the #ifdef/#ifndef code which, due to the amount
of targets and variants, was starting to get a bit out of hand.

Now each acc & gyro is assumed to be used unless the respective
USE_ACC_* and USE_GYRO_* is #undef'd at the top of the file by a
particular target.

This commit also cleans up the mess of re-running all the mpu6050
detection code twice.  Now the acc specific communication code is only
run once and only code that is actually needed to be run twice is run
twice, and the rest of the code is now only used once.  This will
improve board startup time, albeit negligibly.

Sensor alignment is only known for the NAZE target so now the alignment
will be ALIGN_DEFAULT for all targets other than NAZE unless they are
known.

Finally, in the Makefile a file was included twice in the source list
for the STM32F3DISCOVERY target and now additional sensors are supported
for users that may want to try them on that board.