Add an optional global buffer to the command interpreter that can be used for command...

Add an optional global buffer to the command interpreter that can be used for command interpreter output.  This removes the need for multiple output buffers to be allocated when more than one command interpreter is implemented.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@1612 1d2547de-c912-0410-9cb9-b8ca96c0e9e2