Update to efibootmgr 15
    
    - Make efibootmgr use EFIDIR / efibootmgr.efidir like fwupdate does
    - make --loader default build-time configurable
    - sanitize set_mirror()/get_mirror()
    - Add support for parsing loader options as UCS2
    - GCC 7 fixes
    - Don't use -fshort-wchar since we don't run on EFI machines.
    
    Signed-off-by: Peter Jones <pjones@redhat.com>