Adds a C build system for keymaps

[jackhumbert]

Check out the firmware/src/ README.md for more info on the keymaps themselves. In addition to this system, this PR also:

• Fixes the F3 bug introduced in the previous fixes (#13)
• Allows for arbitrary file loading in the updater tool
• Removes the "revised" commands from the updater tool
• Ports the revised changes to the C build system
• Adds custom manufacturing, product, and serial strings to the keyboard (these may need to be decided on from Pine64)
  • If any configuration is sensitive to name changes, you'll need to update that, or change it to use VID/PID
• Starts an arbitrary revision system at 0.1 (which is shown in lsusb -v)