nrf_bsim: Only connect once to Bsim Phy

Each MCU initialization hooks will attempt to call
into the API to connect to the bsim phy and, if the
user requested it thru command line, delay its boot.

Doing this for all MCUs is best to ensure it is done
even if there is no image for the other MCU,
but results in the calls being done twice if there is
2 MCUs images, which results in the simulator API giving
a warning about it being likely an error in the app.

To avoid this problem, let's have this be called
only once.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>