[comboot] Implement INT 22h AX=000Bh (Get Serial Console Configuration)

Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/arch/i386/interface/syslinux/comboot_call.c

 #include <gpxe/init.h>
 #include <gpxe/image.h>
 #include <usr/imgmgmt.h>
+#include "config/console.h"
+#include "config/serial.h"
 
 /** The "SYSLINUX" version string */
 static char __data16_array ( syslinux_version, [] ) = "gPXE " VERSION;
 		break;
 
 	case 0x000B: /* Get Serial Console Configuration */
-		/* FIXME: stub */
+#if defined(CONSOLE_SERIAL) && !defined(COMPRESERVE)
+		ix86->regs.dx = COMCONSOLE;
+		ix86->regs.cx = 115200 / COMSPEED;
+		ix86->regs.bx = 0;
+#else
 		ix86->regs.dx = 0;
+#endif
+
 		ix86->flags &= ~CF;
 		break;