[lkrnprefix] Fix lost command line passed by grub

iPXE specifies a value of 0 for cmdline_size, causing GRUB to not pass
in a command line.  Fix by setting cmdline_size to the maximum value
of 2047.

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/lkrnprefix.S

 pad2:
 	.byte	0, 0, 0
 cmdline_size:
-	.long	0
+	.long	0x7ff
 hardware_subarch:
 	.long	0
 hardware_subarch_data: