rules.mak: cleanup config generation rules

rules.mak: cleanup config generation rules

This addresses two issues with config generation
1. rule generating timestamp has side effect.
Thus cleanup on error does not work.
2. rule for handling timestamp is too generic.
It can create any missing .h file.
As a result when .h file is removed, build
might try to create it using this rule which
results in build errors.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>