hardware tool: define syntax via syntax arg

Summary:
This way the commands syntax is always immediately visible
in the help text.
Help that doesn't help, and
previously the help would essentially just end up telling you to
run --commands to actually get help -.-

The existing syntax argument of addPositionArgument will simply
print the syntax help undearneath the usage line, thus giving
immediate access to information on how to use the application
without first having to go through the --commands argument

(--commands still exists for backwards compat)

Test Plan: -h now useful

Subscribers: broulik, bruns, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26841