[efi] Accept a command line passed to an iPXE image via LoadOptions

Treat a command line passed to iPXE via UEFI LoadOptions as an image
to be registered at startup, as is already done for the .lkrn, .pxe,
and .exe BIOS images.

Originally-implemented-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>