build: Allow building with OpenJDK

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

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

Conflicts:
	core/main.mk

Conflicts:
	core/main.mk

Change-Id: Ie670c9aee204f9c639f78ef963999dcbb023611c