assemble the tool commands at use time, after all

the precise syntax depends on what exactly the command is used for, so
we need to resolve it at the last moment. see followup commits.

This logically reverts commits 6f4ff813
and 731e6bec

.

Change-Id: If285c91d7521069be86d32593b5c2ae2027b3038
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>