openpowerlink: force static library build

There is no shared lib in openpowerlink,
so force static lib to build libpowerlink.a

Fixes:
http://autobuild.buildroot.net/results/b33/b33008d260c95bedc52bed48f191f76843612ef9/build-end.log
http://autobuild.buildroot.net/results/c46/c464bdfb8edaaf69d49ade92ef9cc6f45cc73362/build-end.log



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>