tools: provide --with-system-ipxe

This option lets user specify which binary is to be used as ipxe. If
it is specified, the in-tree ipxe will not be built. This option is in
line with other --with-system-* options we provide.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/config.h.in

@@ -96,6 +96,9 @@
 /* libutil header file name */
 #undef INCLUDE_LIBUTIL_H
 
+/* IPXE path */
+#undef IPXE_PATH
+
 /* OVMF path */
 #undef OVMF_PATH