tools: check for libfdt when building for ARM

libxl is going to want this to aid in the creation of guest device tree blobs.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/config.h.in

@@ -9,6 +9,9 @@
 /* Define to 1 if you have the `crypto' library (-lcrypto). */
 #undef HAVE_LIBCRYPTO
 
+/* Define to 1 if you have the `fdt' library (-lfdt). */
+#undef HAVE_LIBFDT
+
 /* Define to 1 if you have the `yajl' library (-lyajl). */
 #undef HAVE_LIBYAJL