buildscript: clean feeds before updating them

When installing feed packages that already have been installed
from a different feed, OpenWrts feeds script does not change
the feed, but keeps the already installed package.

Therefore, packages are not updated to the correct feed on
updatefeeds, when the package is moved to a different feed
in our buildscript.

The update_feeds function already contains code to clean the already
installed feeds, but only removes the symlinks to the feed directory.
OpenWrts package installation directory (packages/feeds/*) is not
removed, however.

The 'clean' command of the scripts/feeds script properly cleans up
both directories. Hence, the 'rm' command is replaced with it.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

buildscript

@@ -163,7 +163,7 @@ update_feeds() {
     done
 
     echo "cleaning feeds"
-    /bin/rm -rf "$builddir"/feeds
+    "$builddir"/scripts/feeds clean
 
     "$builddir"/scripts/feeds update