Bug#952858: [live-build] frontend help/usage fixme

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221
Owner: jnqnfe@gmail.com

the 'lb' frontend has a fixme for its help/usage strings. 
 - `HELP` does not actually get used anywhere since the only place that
reads it - Help() - is never called.
 - `USAGE` does get called, for instance with `lb --usage`, where you
currently see "FIXME" appear. the top level commands applicable to this
usage string should at least specify the three top level commands and
thus be "lb {clean|config|build}"

alternatively we could consider something like "lb {clean|config|build}
[OPTIONS]", which is especially relevant to `lb config`, but "lb
{clean|config|build}" is at least a good start.

patch to be submitted via salsa shortly (will amend commit with bug
number first)