grub: Add EFI support (improved).

Fixes #110.

This is based on Daniel Hillenbrand's submissions, but it makes the same
adjustments already done in the bootloader module that's already merged:

* Put detect_firmware_type into the grub module itself until a better
  place is found.

* Get the distribution name from the branding configuration and use the
  file_name_sanitizer on it.

* Get the grub-install executable name from the module configuration.

It also fixes a Python syntax error in the original submission.
That's the idea, our suggestion is that the better place for detection is the prepare module, which would check for ```/sys/firmware/efi/efivars``` and load the result into globalstorage early enough for everybody to use.