rename sensor_t to acc_t and gyro_t and remove unused variables from
each.  relocated acc_t/gyro_t from sensors_common.h into
drivers/accgyro_common.h since they define an interface and the
dependency was pointing the wrong way from the drivers.
baro_t/acc_t/gyro_t dependences are all now pointing the right way.
Removing unused temperature member from acc_t.