[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>

src/arch/i386/prefix/lkrnprefix.S

@@ -193,6 +193,9 @@ run_ipxe:
 	movw	%ax, %ss
 	movw	$0x7c00, %sp
 
+	/* Retrieve command-line pointer */
+	movl	%es:cmd_line_ptr, %edx
+
 	/* Install iPXE */
 	call	install
 
@@ -206,6 +209,13 @@ run_ipxe:
 	lret
 	.section ".text16", "awx", @progbits
 1:
+	/* Set up %ds for access to .data16 */
+	movw	%bx, %ds
+
+	/* Store command-line pointer */
+	movl	%edx, cmdline_phys
+
+	/* Run iPXE */
 	pushl	$main
 	pushw	%cs
 	call	prot_call