allow port "auto" config - that enables auto detect of port

[petermm]

Existing behaviour is kept - no defaults changed.

Esptool has auto-detect port features (when --port is left out), where it enumerates serial ports and finds the first port with esp32 connected.

Setting port to "auto" - (in combination with chip auto) - enables huge DX increase - where one can swap devices, and mix atomvm.esp32.flash - without having to figure out and configure the port.

Also avoids funny stuff like a macbook pro having one name for left/right side usb respectively.