[prefix] Add a generic raw image prefix

Provide a generic raw image prefix, which assumes that the iPXE image
has been loaded in its entirety on a paragraph boundary.

The resulting .raw image can be loaded via RPL using an rpld.conf file
such as:

    HOST {
        ethernet = 00:00:00:00:00:00/6;
        FILE {
            path="ipxe.raw";
            load=0x2000;
        };
        execute=0x2000;
    };

Debugged-by: Johannes Heimansberg <git@jhe.dedyn.io>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/prefix/rawprefix.S

@@ -0,0 +1,53 @@
+/*
+ * Raw binary prefix
+ *
+ * Assumes that entire image is already loaded as a contiguous block
+ * on a paragraph boundary and entered in real mode.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
+	.text
+	.arch i386
+	.org 0
+	.code16
+
+#include <librm.h>
+
+	.section ".prefix", "ax", @progbits
+	.globl	_raw_start
+_raw_start:
+
+	/* Adjust %cs so that %cs:0000 is the start of the image */
+	movw	%cs, %ax
+	call	1f
+1:	popw	%bx
+	subw	$1b, %bx
+	shrw	$4, %bx
+	addw	%bx, %ax
+	pushw	%ax
+	pushw	$2f
+	lret
+2:
+	/* Install iPXE */
+	call	install
+
+	/* Set up real-mode stack */
+	movw	%bx, %ss
+	movw	$_estack16, %sp
+
+	/* Jump to .text16 segment */
+	pushw	%ax
+	pushw	$1f
+	lret
+	.section ".text16", "awx", @progbits
+1:
+	/* Run iPXE */
+	virtcall main
+
+	/* Uninstall iPXE */
+	call	uninstall
+
+	/* Boot next device */
+	int $0x18