mesh: Factor out mesh join function

mesh: Factor out mesh join function

Mesh join function consists of two parts which are preparing
configurations and sending join event to the driver. Since physical mesh
join event could happen either right after mesh configuration is done or
after CAC is done in case of DFS channel is used, factor out the
function into two parts to reduce redundant calls.

Signed-off-by: Peter Oh <peter.oh@bowerswilkins.com>