Modbus Master is missing many files and doesn't compile at all. I'm attaching diff with my implementation. More details in https://groups.yahoo.com/neo/groups/nuttx/conversations/topics/13734

apps/nshlib: nsh_getdirpath(), use snprint instead of sprintf to avoid possibility of buffer overrun.  Noted by Chung Hwan Kim.

Add a new buttons example that uses the button character driver instead of the architecture buttons directly

apps/nshlib: Add printf command to NSH, e.g., controlling /dev/userleds from command line: nsh> printf \x01 > /dev/userleds