MPU6000: Use signed addition when accumulating readings.

- Fixes very erratic accel readings on the PX4 board.
MPU6000: Flip Y and Z axes for SMACCM_HAL.

- The accelerometer is upside-down on the PX4FMU vs the APM2.
AP_HAL_SMACCM: Implement RCInput driver.
MS5611: Fix CONFIG_MS5611_SERIAL definitions.

- Make sure the values are defined as integers.  We were always using
  SPI rather than using I2C when desired.
ArduCopter: Add AP_HAL_SMACCM support.

- Added default configuration to "config.h".
- Added main function to run under FreeRTOS with HWF4.
AP_HAL_SMACCM: Fix pin conflict between uartC and I2C.

- uartC is currently disconnected.
- Set uartB to USART6 for GPS communication.
AP_HAL_SMACCM: Increase scheduler stack sizes.