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>
Some changes added to compile and run with Java 6 and Java 7.

  - correction of errors with Javac 6:
    - target 1.5 and source empty.
  - correction of errors with Javadoc 7:
    - strange behavior when duplicate classes are given.

Change-Id: Ib814ede143f7799ef18e0fb1b733547519b8b0e5
Allow building with javac 1.7

Change-Id: Id8d78558736482e10c52f2cc86a2b903b1fbdbe5