wic: Add dummy subcommand and usage strings

wic: Add dummy subcommand and usage strings

In order to reuse the existing subcommand infrastructure to display
various general-purpose help topics, add a dummy 'help_topic'
subcommand and usage string.  This allows users to invoke general help
topics by the natural form 'wic help <topic>' even though topic
doesn't correspond to a real subcommand.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>