PRU firmware for HC-SR04 rangefinder

HC-SR04 driver that can be used with BeagleBone Black

Install PRU C-Compiler

1. sudo apt-get update
2. sudo apt-get install ti-pru-cgt-installer

Rebuild rangefinder.c

1. cd ardupilot/Tools/Linux_HAL_Essentials/rangefinderpru/
2. make