Fix generation of the rpi-firmware and rpi-userland version files

They weren't being updated correctly, and they shouldn't have been
stored in git in the first place
Merge pull request #68 from lurch/fix-version-files

Fix generation of the rpi-firmware and rpi-userland version files
Provisional fix for #70 (requires testing). Also adds ability to create
missing MBR and warn users to reinstall NOOBS.
NOOBS 1.2.1 release for testing (fixes #70 by adding FAT partition label
with mtools instead of dosfslabel).
Update translation files from latest text strings