libkcapi: split BR2_PACKAGE_LIBKCAPI_APPS

Since commit https://github.com/smuellerDD/libkcapi/commit/af313accf18887b4f6dbd7306c2389ddf271c9b7,
hasher test application depends on dlfcn.h. To avoid putting all apps
under !BR2_STATIC_LIBS dependency, split BR2_PACKAGE_LIBKCAPI_APPS into
several variables.

Moreover, BR2_USE_MMU is only a dependency of BR2_PACKAGE_LIBKCAPI_TEST
as other applications don't use fork.

Finally, enc or dgst applications could be enabled in a next patch if
needed.

Fixes:
 - http://autobuild.buildroot.net/results/7c57f3025030eff41a5cde52759821249859caf1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>