We were using USER_CLASS loading to allow conditional checking
on DISTRO_FEATURES, which triggered distro feature specific version
pinning.

It was found that DISTRO_FEATURES set in local.conf is not
consistently available at layer.conf parse time, hence our checks
were not always working as expected (i.e. the version files are
not included).

If we move the DISTRO_FEATURE check to the bbclasses, and use it
to trigger the include, we should have a consistent set of variable
resolution and consistent behaviour.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>