radvd: not available on AVR32

radvd needs some C library definitions not available in old uClibc
versions. Since we don't care much about AVR32, we simply disallow the
radvd package on this architecture.

Fixes:

  http://autobuild.buildroot.org/results/f03/f0367a30e8d772eb5e955a85b853ec3e43fe54b3/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>