Add warning message for java6 builds.

The instruction URL has been amended and a few additional
details have been added.

(cherry picked from 8c06afdea3d3c422d653b40fd19d5e3d4be64af2)

Change-Id: Icaffc3b13ed881ac7e29f2021ed31eb1f877a5ab

Conflicts:
	core/main.mk