uefi: Don't overwrite CustomFlags if set from a quirk
uefi: Don't overwrite CustomFlags if set from a quirk

hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd