[oe] meta-java configuration

[Kyle Russell]

It looks like the only javac that meta-java exposes through its bbclasses
comes from libecj-bootstrap-native 3.6.2.  A few years ago, it looked like
there was an attempt to allow openjdk-8-native to provide
virtual/javac-native, but it has since been reverted.

I think I understand libecj is needed as part of the bootstrap process to
build openjdk, but if other layers want to build newer java code using
javac from openjdk 8 once available after bootstrap, does meta-java provide
support for using javac from openjdk-8-native, or is the expectation that
other layers would depend directly on openjdk-8-native and setup
EXTRANATIVEPATH as needed to reach its tools?

Maybe this is planned future work, but I wanted to make sure I was
configuring the layer correctly and using it as intended.

Thanks!

Kyle