[build] Allow APPEND lines in ipxe.iso to function as expected

Signed-off-by: Dominic Cleal <dcleal@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/geniso

 	g=${g//[^a-z0-9]}.krn
 	case "$first" in
 	"")
-		echo DEFAULT $g
+		echo DEFAULT $b
 		;;
 	esac
 	first=$g