nrf52_bsim: Add options to synchronize to the Phy before booting

Some testcases may benefit from having options to synchronize
to the Phy either before running the test pre-initialization
and/or before booting the CPU.
Provide these options.

Background: Before synchronizing to the Phy devices execute
ahead asynchronously. This is not a problem in most cases,
as normally the only interaction between devices happens thru the
radio, and as soon as any radio interaction occurs, the devices
are step-locked by the Phy.
But some devices do share information in other ways between,
like for example using the backchannels. This may benefit
from using these options.

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