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.
The bootloader module and the grub module share a lot of code. They do the exact same thing on BIOS systems, the difference is the bootloader used for EFI. So the idea was to put this here as long as we haven't decided on a better place to put the detection.