configure: add hint of libfdt to DTC dependency not found message

Most distros package it as libfdt, and mentioning libfdt here makes it
much easier to find the package you're missing.
Signed-off-by: NStewart Smith <stewart@linux.vnet.ibm.com>
Reviewed-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -2662,8 +2662,8 @@ EOF
     fdt_libs="-L\$(BUILD_DIR)/dtc/libfdt $fdt_libs"
   elif test "$fdt" = "yes" ; then
     # have neither and want - prompt for system/submodule install
-    error_exit "DTC not present. Your options:" \
-        "  (1) Preferred: Install the DTC devel package" \
+    error_exit "DTC (libfdt) not present. Your options:" \
+        "  (1) Preferred: Install the DTC (libfdt) devel package" \
         "  (2) Fetch the DTC submodule, using:" \
         "      git submodule update --init dtc"
   else