rt-tests: disable on bfin

rt-tests depends on NPTL which is not available on the bfin
architecture.

Fixes:
  http://autobuild.buildroot.net/results/ac9/ac95f73b2351d46d9a8c48caae7f6c3ea124e8bc



Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>