Add .gitignore file

Ignores generated bitmaps, objects, binaries, and MacOS specific files.
Create a upload_mac target

Uploads firmware on a Mac. Tested, seems to be working fine. Your mileage may vary
but it's pretty much impossible to brick this device since it uses the SBL feature of the
STM32...
Merge pull request #21 from elafargue/master

Infrastructure: create .gitignore file, and create a Mac firmware upload target