Extend CDC/ACM driver so that can be connected/disconnected under software...

Extend CDC/ACM driver so that can be connected/disconnected under software control; Add new NSH commands sercon and serdis that will connect and disconnect the CDC/ACM serial device

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@4436 42af7a65-404d-4744-a932-0658087f49c3