RFC2217 connections; boardname: prefix in paths

[milung]

• Added support for opening RFC2217 remote serials (see e.g. https://docs.espressif.com/projects/esptool/en/latest/esp32/esptool/remote-serial-ports.html)
• To enforce specific device path, the path may be prefixed with ":", e.g. ls pyboard:/flash, or ls : for default board