configure: append --extra-ldflags to LDFLAGS

The help text says --extra-ldflags is appended to LDFLAGS so make it so.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configure

@@ -355,7 +355,7 @@ for opt do
   --extra-cflags=*) QEMU_CFLAGS="$QEMU_CFLAGS $optarg"
                     EXTRA_CFLAGS="$optarg"
   ;;
-  --extra-ldflags=*) LDFLAGS="$optarg $LDFLAGS"
+  --extra-ldflags=*) LDFLAGS="$LDFLAGS $optarg"
                      EXTRA_LDFLAGS="$optarg"
   ;;
   --enable-debug-info) debug_info="yes"