packages/fff: Only use PKG_RELEASE for our own packages

PKG_VERSION is meant to refer to the version of external packages,
as we do e.g. in the tunneldigger package.

For our own packages, we just need the PKG_RELEASE variable.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
[Rebased onto current state of master]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>