Support for java6 and java7 on android-build

Hello,

Last Friday, we with Vishal made changes to build scripts on https://android-build.linaro.org/ to support both java-1.6-sun (as was required by previous AOSP versions) and java-1.7-openjdk (as should be required for AOSP master). (Based on https://bugs.launchpad.net/linaro-android-infrastructure/+bug/1310939)

To explicitly select java6, add "USE_JAVA_6=1" to build config. For java7 respectively add "USE_JAVA_7=1". The default stays java6 so far, as it appears to be needed by the most jobs so far, but it may be changes in the future.

Thanks, and let me know if there're any issues or questions.