Fix error finding boards.txt at compile time

Added ARDUINO_DIR to makefile explicitly, to make sure the make process
can find boards.txt even if the environment variable is unset.
Enable GUI-driven build and monitor actions

Update to the script structure, primarily to enable right-click menus
in Nautilus that build (and open a serial terminal on) the selected sketch.
Add README and xdg-open files to project

Create a README, and attached the xdg-open file (which integrates with
the GUI enhancements added last commit) to the project.