The help command currently only lists available commands. It was asked
by some developers some time ago, and this implementation only costs
150 bytes in the final exectuable.  I don't think it's worth a Kconfig
option.

This is it in action with my configuration:

   wrc# help
   Available commands:
     ver
     pll
     sfp
     stat
     ptp
     mode
     calibration
     time
     gui
     sdb
     mac
     init
     ptrack
     ip
     verbose
     help

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>