AP_InertialSensor: added L3G4200D driver

this is for a cheap ebay 10DOF sensor
AP_InertialSensor: added L3G4200D example
DataFlash: prevent valgrind errors from Empty driver
HAL_Linux: better realtime setup

lock all memory and pre-fault the stack in all threads
HAL_Linux: don't prevent write while timer is busy

the timer can block on I2C writes, which causes the main loop to run
AP_Common: added product ID for L3G4200D
AP_Compass: prevent NULL deref on enable mag while running
AP_Compass: fixed orientation on Linux
AP_Baro: fixed I2C semaphore handling for BMP085 driver
HAL_Linux: improved latency of timed semaphores
Plane: switched to L3G4200D driver for Linux
Plane: improved latency of main loop

use a 500usec delay
Rover: switched to L3G4200D driver for Linux
Rover: improved loop() latency
AP_Compass: don't wait more than 1ms for compass sample

this prevents 5ms delays in compass accumulate
AP_InertialSensor: added a small delay in accel calibration

allows threads to run on Linux
build: added -lrt for Linux build
Copter: switch Linux to L3G4200D ins driver