build: Allow building with OpenJDK

Change-Id: I6348bc1187240729f75f50d7ef91802c68e7788b
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

Conflicts:

	core/main.mk

Change-Id: Ief980703f0ff747166c1ec79ad748c09d5979c01