tools: make config-gen to require build target argument

When argument is not supplied help user by telling what arguments can be
used.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: check commands needed for running a test exist

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>