PX4: mkblctrl support and basic LED support added

I added the support for mkblctrl (Mikrokopter i2c ESC) support in the rc.APM script.
To enable the mkblctrl driver add a empty file with the name mkblctrl to the APM directory on the microsd card.

As a second option i added support to support mavlink on the /dev/ttyS0 device.
To enable it, just place a empty file with the name mavlink-ttys0 into the APM directory.

Basic LED support to the PX4_HAL was added. 
Blue LED is for GPS
Red LED is for Arming

This changes where tested and flown.