HAL_Linux: include readRegistersMultiple in I2CDriver

AP_HAL_Linux implemments this new method.
AP_InertialSensor_L3G4200D: Clock-based wait_for_sample() impl.
AP_Motors_test: Adapt to test bench available
AP_Notify: added pins for Linux port
HAL_Linux: added initial gpio library
HAL_Linux: add export gpio during direction set if not exported during initialisation
HAL_Linux: increase size of sysfs_path variables inside LinuxRCOutput's methods

to avoid stack smashing error
HAL_Linux: Add GPIO pinmux mappings

This will allow to request channels using the
name of the BBB headers (e.g. P8_12, P9_42).
HAL_Linux: add support for LinuxDigitalSource in AP_HAL_Linux
GPS_UBLOX: fix test to work with AP_HAL_Linux.

Some libraries were missing.
AP_InertialSensor_MPU6000: Correct DRDY pin.
AP_InertialSensor_MPU6000: Remove MPU6000_CS_PIN

Remove MPU6000_CS_PIN, a legacy way of handling
Chip Select.
AP_InertialSensor_MPU9250.cpp: Update the DRDY pin.
AP_InertialSensor_MPU9250: remove legacy CS.
HAL_Linux: change to mem access method from sysfs access method

This step is taken to reduce the time for gpio access substantially
HAL_Linux: spidev1.0 to SPI0 and spidev2.0 to SPI1
AP_InertialSensor_MPU9250: Add missing library for example/

In order to compile the examples test a library needed to
added.
Tools: Add PXF enable and check CS scripts
AP_InertialSensor: Add missing library to test example.
HAL_Linux: Add CS capabilities to the SPI driver.