Use fast speed in HWF4 I2C driver.
Use semaphores instead of suspending timer procs in AP_Baro.
Fix inconsistent results from timer_get_ticks.

- There was an edge case where, if we read the ticks right as the
  timer rolled over but before "g_ticks" was updated, we'd get a
  bogus value.
Remove ap_hal subdirectory.

- Preparation for building from the ArduPilot source tree rather
  than forking the code into this repository.
Build with out-of-tree ArduPilot source.

- Added build system and sensor test.
- Add CONFIG_ARDUPILOT_PREFIX to Config.mk to point to the root
  of the ArduPilot source tree.
- Building ArduPilot sources is currently optional.  If the config
  setting is not defined, do not try to compile it.