Currently the ports for flashing such as /dev/ttyACM0, etc are hardcoded in the makefile. These should be changed to environment variables specified when running make, e.g:
PORT=/dev/ttyACM1 make upload-rak4631

There should also be a default port specified for each upload target if the environment variable isn't set. These should all be set under the assumption of Linux port naming conventions, e.g. /dev/ttyACM5, etc.