Config.mk: replace dependency to genpath with actual target

Config.mk: replace dependency to genpath with actual target

genpath is a detail of buildmakevars2file. Replace the dependency to
genpath with the actual buildmakevars2file target. This change by
itself does not fix any bug. Upcoming changes will add dependencies to
$(target), but no rule exist to create $(target).

To force a rebuild of the $(1) rule the target now depends on the
existing .phony target. This dummy target is already used elsewhere in
the code.

No change in behaviour is expected by this patch.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>