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
Use -mcpu=cortex-a15 for krait

Based on https://android.googlesource.com/platform/build/+/d195c6d14e027f12f63508afed245a1ce469c936

Change-Id: Ib409c915c5302cdb44d49978456b7d1cc6a2a250
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>