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>
botan: add type to BR2_PACKAGE_BOTAN_ARCH_SUPPORTS option

Commit 1737af367993326ee47e847bd755806c8a78ced8 ('botan: limit to
supported CPU architectures') introduced the
BR2_PACKAGE_BOTAN_ARCH_SUPPORTS with a default value, but forgot to
define a type, which leads to a kconfig warning:

  package/botan/Config.in:1:warning: config symbol defined without type

This commit fixes that by properly setting the type.

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