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.
Also rebuild for efivar-31-1.fc26 to get symbol versioning right.
Resolves: rhbz#1468841
Signed-off-by: Peter Jones <pjones@redhat.com>