[build] Fix default target in sdsk image

gensdsk currently creates a syslinux.cfg file that is invalid if the
filename ends in lkrn.  Fix by setting the default target to label($b)
instead of filename($g).

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

src/util/gensdsk

@@ -48,7 +48,7 @@ do
 	g=${g:0:8}.krn
 	case "$first" in
 	"")
-		echo DEFAULT $g
+		echo DEFAULT $b
 		;;
 	esac
 	first=$g