botan: limit to supported CPU architectures

See src/build-data/arch/ for the list of supported CPU architectures.

Fixes:

  http://autobuild.buildroot.org/results/69f/69fdecb06a03e6a95adfab70adf8ebf45971f0da/



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