libcap-ng: disable on avr32 which lacks TLS

libcap-ng needs TLS support, which is not available on avr32.
Also, disable reverse dependencies.

Fixes:
  http://autobuild.buildroot.net/results/73c/73c7c211a51c312bbe4eb6a540f3ad9c92c79ebe/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>