efi: Update the URLs for efibootmgr

Matt Domsch changed the dell page to point to the new upstream quite
some time ago; kernel should reflect that here as well.

Cc: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>