yocto-check-layer: Only note a layer without a conf/layer.conf (versus error)

Dynamic layers may have a conf directory, but don't need (or
want) a conf/layer.conf

This isn't an error, so we can just log it and indicate the
layer is being skipped.

A full layer without a conf file isn't all that useable, so we
aren't letting anything subtle slip through by just logging
it.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>