hg: build-infra/jdk8: Fix broken multiple configuration building (CONF=x).

[magnus.ihse.bursie at oracle.com]

Changeset: 0baa756d4293
Author:    ihse
Date:      2012-09-26 14:25 +0200
URL:       http://hg.openjdk.java.net/build-infra/jdk8/rev/0baa756d4293

Fix broken multiple configuration building (CONF=x).
Autodetect top-level make targets so dispatching with CONF=x works better
even when adding new top-level make targets.
Also fix missing clean targets and improve output when building with CONF=x.

! common/makefiles/MakeHelpers.gmk
! common/makefiles/Makefile