build: Allow building with OpenJDK v1.7

Ubuntu-12.10 ships with OpenJDK v1.7 and AOSP errors out if you try
to build it with JAVA version other than 1.6. Linaro Android build
with JAVA v1.7, boot tested on Origen 4412, seems to work fine.

Change-Id: I8ca783b48de80131f830139a33d8fc0aa92c562c

Conflicts:
	core/main.mk