Previously, ports that were needed for a scenario and that were not
prepared or outdated, triggered one assertion each during the second
build stage. The commit slots a mechanism in ahead that gathers all
these ports during the first build stage and reports them in form of a
list before the second build stage is entered. This list can be used
directly as argument for tool/ports/prepare_port to prepare respectively
update the ports. If, however, this mechanism is not available, for
example because a target is build without the first build stage, the old
assertion still prevents the target from running into troubles with a
missing port.
Fixes#1872
The new 'select_from_ports' function allows a target description file to
query the path to an installed port. All ports are stored in a central
location specified as CONTRIB_DIR. By default, CONTRIB_DIR is defined
as '<genode-dir>/contrib'. Ports of 3rd-party source code are managed
using the tools at '<genode-dir>/tool/ports/'.
Issue #1082
