[sys_time] Proper definition of sys_time_usleep
[imu] update GX3 driver

using fixed usleep from sys_time
[chibios] Board config files renamed more appropriately
Merge branch 'rt_paparazzi' of git://github.com/paparazzi/paparazzi into rt_paparazzi

Conflicts:
	sw/airborne/subsystems/imu/imu_gx3.h