AP_HAL_FLYMAPLE: Sensor reading improvements

I2C speed increased to 400kHz
Accelerometer 8g max, Normal power, 800Hz bandwidth
Gyros: 2000 degrees/sec, 256Hz LPF, 8kHz internal sample rate
AP_HAL_FLYMAPLE: UARTDriver libmaple internal ring buffers enlarge on demand
AP_InertialSensor: Formalise and document Flymaple Gyro scaling factor
AP_InertialSensor: Added examples/Flymaple for testing
AP_InertialSensor: Flymaple now polls sensors at 800Hz. More changes to
come...
AP_InertialSensor: Flymaple sensors improvements

Flymaple sensors are now raw sampled at 800Hz, and LowPassFilter2p is used
to filter to the desired filter cutoff frequency.
Flymaple sensor polling is done in the main thread NOT in the timer
interrupts.
AP_InertialSensor: mark flymaple as nobuild

won't build with build_examples.sh script
AP_HAL_Linux: initial Linux AP_HAL
AP_HAL: added AP_HAL_LINUX define
AP_Compass: added HAL_Linux support
AP_Notify: blank Linux LED defines
AP_Progmem: Linux doesn't use progmem
build: added Linux targets
Plane: added HAL_LINUX includes
AP_Progmem: only AVR uses progmem

no need to list all other boards
AP_HAL_Linux: initial I2C driver implementation

this works sufficiently for the HMC5883
AP_Compass: fixed some old Serial.print calls
AP_Compass: added Linux support to the compass test
AP_Compass: neaten up HMC5883 debug
HAL_Linux: switched to pthread based scheduling