configure: postfix --extra-cflags to QEMU_CFLAGS

It makes sense that extra-cflags should be appended after the normal
CFLAGS so they don't get overridden by default behaviour. This way if
you specify something like:

  ./configure --extra-cflags="-O0"

You will see the requested behaviour.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -353,7 +353,7 @@ for opt do
   ;;
   --cpu=*) cpu="$optarg"
   ;;
-  --extra-cflags=*) QEMU_CFLAGS="$optarg $QEMU_CFLAGS"
+  --extra-cflags=*) QEMU_CFLAGS="$QEMU_CFLAGS $optarg"
                     EXTRA_CFLAGS="$optarg"
   ;;
   --extra-ldflags=*) LDFLAGS="$optarg $LDFLAGS"