The primary firmware source code for USB HackRF devices is hackrf_usb.  Most of
the other directories contain firmware source code for test and development.
The common directory contains source code shared by multiple HackRF firmware
projects.

The firmware is set up for compilation with the GCC toolchain available here:

https://code.launchpad.net/gcc-arm-embedded

Required dependency:

https://github.com/mossmann/libopencm3