Add support for use of a USB serial device to provide NSH console I/O. ...

Add support for use of a USB serial device to provide NSH console I/O.  Verified on the Sure PIPIC32MX board

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@4458 42af7a65-404d-4744-a932-0658087f49c3