esp8266/scripts/webrepl: Add "first connection" mode to setup password.

If there's no port_config.py file, or it lacks WEBREPL_PASS variable,
"initial setup mode" will be entered on first WebREPLconnection. User
will be asked for password, which will be written to
port_config.WEBREPL_PASS, and system restarted to work in normal mode
with password active.