Boot JDK used with jdk7 builds

[Kelly O'Hair]

The documented Boot JDK to be used when building JDK7 repositories is JDK6 Update 18, as listed here
  http://hg.openjdk.java.net/jdk7/build/raw-file/tip/README-builds.html#MBE

However, as many people know, JDK releases newer than JDK6u18 should always work, and some older ones can work too.
Variations in build dependencies can sometimes create unwanted differences between builds by different developers
or build systems, so our tendency is to minimize these variations, but by doing so, we run a risk that we don't
see what does not work. 

If you have any incidents where particular JDKs can or cannot be used as the Boot JDK for JDK7, please let us know.

-kto