efilinux: Make option parsing more robust

Thomas reports that efilinux hangs when executed via gummiboot. The
reason being that efilinux expects the executable name to be prepended
to the beginning of the LoadOptions string.

This behaviour isn't required by the UEFI spec, so make the options
parsing more robust to handle either case.

Reported-by: Thomas Bächler <thomas@archlinux.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>