[lkrnprefix] Make real-mode setup code relocatable

The bzImage boot protocol allows the real-mode code to be loaded at
any segment within base memory.  (The fact that both iPXE and recent
versions of Syslinux will load the real-mode code at 1000:0000 is a
coincidence; it is not guaranteed by the specification.)

Fix by making the code relocatable.

Reported-by: Andrew Stuart <andrew@shopcusa.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/lkrnprefix.S

 FILE_LICENCE ( GPL_ANY )
 
-#define BZI_RM_SEGMENT 0x1000
 #define BZI_LOAD_HIGH_ADDR 0x100000
 
 	.text
 realmode_swtch:
 	.long	0
 start_sys:
-	.word	BZI_RM_SEGMENT
+	.word	0
 kernel_version:
 	.word	version_string - 0x200
 type_of_loader:
 
 setup:
 	/* Fix up code segment */
-	ljmp	$BZI_RM_SEGMENT, $1f
+	pushw	%ds
+	pushw	$1f
+	lret
 1:
 	/* Set up stack just below 0x7c00 and clear direction flag */
 	xorw	%ax, %ax