Include possibility of covering of boot-classpath

Many projects are runnig some of theirs parts in boot classapth for various reasons.
Right now jacoco is exuding boot classpath explicitly.

I think it is worthy to include possibility to cover also bootclassapth, however with strong documentation (as some parts, eg jacoco or java.lang must be excluded otherwise fatal error or endless loop occure )and especially with "use on your own risk" :)

For my own purposes I have added this behaviour to jacoco -
http://mail.openjdk.java.net/pipermail/distro-pkg-dev/attachments/20121127/ba8f6a1e/xboot-0001.patch (for reasons - http://mail.openjdk.java.net/pipermail/distro-pkg-dev/2012-November/020984.html) .
This proof of concept is very simple, it s adding new agent switch xboot=true/false default si false, and let user to include/exclude what he needs. Then it is just not excluding the boot classapth.

I think this improvement can be fast-done one and will bring enormous benefits. From above proof-of-concept i can confirm it is working, as I have successfully covered several Xbootclassapth misusing projects.