configure: Fix "ERROR: ERROR: " for missing/incompatible DTC

error_exit already prepends "ERROR: ", so no need to print it manually.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -2529,7 +2529,7 @@ 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 "ERROR: DTC not present. Your options:" \
+    error_exit "DTC not present. Your options:" \
         "  (1) Preferred: Install the DTC devel package" \
         "  (2) Fetch the DTC submodule, using:" \
         "      git submodule update --init dtc"