decouple cli/msp from each other.  relocated non-msp code into
serial_common.c/h. decouple runtime_config from serial ports.  decouple
buzzer from serial ports.  decouple opening of the main serial port from
the msp code.  decouple serial rx providers from runtime_config.  rename
core_t to serialPorts_t since it only contained serial ports.  It's now
clear which files use serial ports based on the header files they
include.