AP_Baro: ask for maximum poll rate for PX4
HAL_PX4: added APM startup script
build: added EXTERNAL_SCRIPTS to PX4 build
AP_Mission: Command and waypoint handling library.

- Reads and writes MAVLINK commands to/from storage.
- Contains a waypoint queue of previous, current, and after waypoints.
- Provides commands associated with current mission leg.
- Allows incrementing waypoints and changing to specific waypoints at anytime.
- Handles DO_JUMP commands while maintaining a valid waypoint queue.
- Performs error checking on waypoint values.
AP_Mission: Removed cliSerial and uncrustified files
AP_Mission: Addition of command_index() function, that just returns the current index (_index[1])