auto detect correct serial port for repl and flashing

[mattvenn]

there is too much room for error with the makefiles and mount points

perhaps mount points stay the same but it's very common for the serial ports to change names, especially as I have to unplug and replug the board every time I try to use it

so better to detect which is the correct port and set it automatically

[jeffdi]

the PATH is automatically set. the mount point still needs to be set by the user.