I've refactored the use of the serial port, to store the
current serial port in a global variable in a module.
This is set to the most recent port to receive data,
and so we should always be replying to the port
that was used to initiate a command.
If SECONDARY_SERIAL_PORT is not defined, this
should result in no change - if it is, we should be
able to communicate on either port.
HW serial temporarily broken due to the change in pico core. This core
has a lot more features e.g. easily changing PWM frequency and supports
both pico cores (currently unused)
