We previously had the modem initialization command merged with some other port
setup commands in the 'modem_init' step of the 'Modem' interface. Instead of
doing this, we now split the logic into two separate steps:
A first 'enabling_modem_init' modem initialization step is to be run just after
the ports have been opened, but only during the first enabling operation, and
only if the modem was not hotplugged. A hotplugged modem is assumed to be
properly initialized already, so no need to ATZ-it. Also, we will now wait 500ms
by default after the modem initialization command has been sent, to let it
settle down.
The second 'modem_init' step will be run during the 'Modem' interface
initialization, and it currently only holds specific setup of the primary and
secondary serial ports. We'll be modifying this logic a bit in the next commits,
so no big deal to have that step name unchanged.
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