cookerdata: Convert multiconfig to use BB_CURRENT_MC

cookerdata: Convert multiconfig to use BB_CURRENT_MC

People are struggling with multiconfig as the point the conf file
is injected into the data store is not what people expect. We can't
really use a post config since that is too late and we can't really
use a pre config file since that is too early. In OE terms, we need
something right around the local.conf point so it behaves in a similar
way.

A way to handle this is to set the new variable BB_CURRENT_MC to be the
currently selected multiconfig, then the metadata itself can choose
when to inject the approriate configuration.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>