build: Allow building with OpenJDK

My local builds with OpenJDK have been successful - no
reason to disallow it, let's go free...

Also allow OpenJDK 7

Conflicts:
	core/main.mk

Change-Id: I01e6cb87e8a7f39db7d762405889221ce287f1ef
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>