GPS over EXT serial only release

This is a build to convert a DFM17 into a GPS dongle

Some assembly required

To use this, it's as simple as chasing a DFM, soldering wires to the debug pins and uploading this binary using the ST-Link utility.
When you plug in the sonde to a usb serial adapter, you'll see version info and some blank NEMA messages until a fix is acquired.