AP_HAL_Namespace: Add I2CDevice global names.

This names will be used for the multi-bus aware
I2CDriver. Each sensor will need to pass its name
as a parameter so that the I2CDriver can remember
in which bus they where.
HAL_Linux_Class: Remove the global semaphore.

Remove the global semaphore from the I2CDriver
class. In the Multi-bus aware I2CDriver, there
is one semphaore per i2c bus.

It's implemmented this way so that both busses
can be used simultaneously.
I2CDriver: multi-bus aware driver.

Lock_bus and release_bus methods added to the
class to select. These two calls should be
used in the AP_HAL_Linux I2C drivers instead
of semaphore->take().
AP_InertialSensor_L3G4200D: Adapt to the multi-bus I2CDriver.