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.
ArduCopter: Use 16-bit arithmetic when comparing event tick counters.

- On the ARM, once the tick counter wrapped, we stopped running events
  because the wraparound case wasn't being handled correctly.  Make
  sure the comparison is 16 bits to prevent this.
mk: wibbles clean up output a little

clean up extra slash in /mk//Arduino.mk
etags can be .PHONY so we dont get errors about overriding existing recipie
AP_HAL_SMACCM: remove meaningless example dir
ArduCopter Makefile: Don't include targets.mk directly (included by apm.mk)
Rover Makefile: don't include targets.mk directly (included by apm.mk)
Arduino.mk: more verbose default config file, with default HAL_BOARD apm2
AP_HAL_AVR: UARTDriver guard needs AP_HAL_Boards.h
AP_HAL_SMACCM: guard compilation on CONFIG_HAL_BOARD