Add a minimalist implemention of the 'cu' terminal program (part of Taylor...

Add a minimalist implemention  of the 'cu' terminal program (part of Taylor UUCP for ages). Using it, you can simply open a serial port and interact with it. Using '~.' you can leave the terminal program and drop back to nsh.

This might  come in handy for other people that have e.g. GSM modems, GPS receivers or other devices with text based serial communications attached to their Nuttx systems.