commands/efi/efifwsetup: Print an error if boot to firmware setup is not supported

commands/efi/efifwsetup: Print an error if boot to firmware setup is not supported

The "fwsetup" command is only registered if the firmware supports booting
to the firmware setup UI. But it could be possible that the GRUB config
already contains a "fwsetup" entry, because it was generated in a machine
that has support for this feature.

To prevent users getting an error like:

    error: ../../grub-core/script/function.c:109:can't find command `fwsetup'.

if it is not supported by the firmware, let's just always register the
command but print a more accurate message if the firmware doesn't
support this option.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>