net: openthread: Fix stack overflow for joiner

Fixed stack being to small for joiner operations.
Enabled auto joining even in case of manual start.
Fixed attachement of SED on norfic radios.

Signed-off-by: Marek Porwisz <marek.porwisz@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>