Add FrSky telemetry support

This adds support to send telemetry data to an attached FrSky receiver.

```
usage: frsky_telemetry start [-d <devicename>]
       frsky_telemetry stop
       frsky_telemetry status
```

The new `frsky_telemetry` app emulates a FrSky sensor hub ([FSH-01](http://www.frsky-rc.com/product/pro.php?pro_id=103)) by periodically sending data packets on the specified UART device.
It currently supports the following data:
* Every 200ms:
    * acceleration values (X-Y-Z)
    * barometer altitude (vario)
    * temperature
    * battery voltage & current
* Every second:
    * course
    * latitude
    * longitude
    * ground speed
    * altitude (GPS)
    * remaining battery level
* Every five seconds:
    * GPS date & time

To connect a FrSky receiver to a PX4FMU UART, a TTL-to-RS232-converter is required. The following example shows the wiring to connect a D4R-II receiver using a FrSky FUL-1 converter:
![PX4FMU to D4R-II wiring](http://i.imgur.com/GqXr0Wd.png)

To start the `frsky_telemetry` app on PX4FMU UART5 on startup, create a file called `/etc/rc.local` on your SD card with the following contents:
```
# Start FrSky telemetry on UART5 (ttyS2)
frsky_telemetry start -d /dev/ttyS2
```