boards: control the network CPU directly

Board initialization code should not use the network CPU management. It
acts as a permanent request for the network CPU, which causes its users
to be unable to turn it off. Instead, let the board initialization code
control the network CPU directly. It sets initial state of the network
core but does not have any impact on sharing the network CPU as a
resource between its users.

Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>