Build system and git versioning

This PR adds a simple Makefile-based automation to build our firmware images. The default version was changed to a new nightly scheme that includes an abbreviated git version (i.e. `nightly~20141109~29ace1c`). If the current commit is tagged, the tag name will be used as release version.

The file `Readme.md` was renamed to the canonical name `README.md` and updated with build instructions.