Remove useServos from core_t since it's only needed for initial startup
and by the mixer.  main.c now asks the mixer if it needs to use servos
and the mixer decides this in mixerInit().  Main already has a
dependency on the mixer.  This allows future commits to further clean up
core_t which is still a source of dependency problems.
Remove mpu6050_scale from core_t by providing a sensor independent way
of determining a revision code.  Previously there was mpu6050 specific
code in cli.c (the status command).

Finally this commit has removed all non-serial port configuration
settings from core_t so that a future commit can refactor core_t to
reduce dependencies on serial port code.

In doing this I also noted from other source code that the MPU6050
accelerometer trim for some revisions appeared to be incorrectly set to
255 * 8 instead of 256 * 8.