Rebuild alambre lib if our embedded git repo changes HEAD.

This causes the lib to get rebuilt e.g. if we git pull inside the repo,
or switch to a branch, or anything else that changes which branch/commit
the work tree refers to.
Pass -mmcu argument when building for arduino_uno target.
Finish the wiring code for the arduino_uno target.

This uses a custom implementation of mapping from 2D to 1D that takes
into account the bizarre shape I cut my 160-pixel LED strip into, with
zig-zagging rows and two rows shorter than the others.