grub: Add EFI support.

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.
grub/test.yaml: Define the shortProductName.
Move detect_firmware_type to the prepare module.

This avoids the duplication between the bootloader and grub modules.
Merge pull request #156 from calamares/grub-efi-support-rfe-110

grub: Add EFI support (improved).