[prefix] Allow iPXE's own command line to be executed as a script

[prefix] Allow iPXE's own command line to be executed as a script

Some prefixes (e.g. .lkrn) allow a command line to be passed in to
iPXE.  At present, this command line is ignored.

If a command line is provided, treat it as an embedded script (without
an explicit "#!ipxe" magic marker).  This allows for patterns of
invocation such as

  title  iPXE
  kernel /boot/ipxe.lkrn dhcp && \
         sanboot iscsi:10.0.4.1::::iqn.2010-04.org.ipxe.dolphin:storage

Here GRUB is instructed to load ipxe.lkrn with an embedded script
equivalent to

  #!ipxe
  dhcp
  sanboot iscsi:10.0.4.1::::iqn.2010-04.org.ipxe.dolphin:storage

This can be used to effectively vary the embedded script without
having to rebuild ipxe.lkrn.

Originally-implemented-by: Dave Hansen <dave@sr71.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>