[oe] [meta-java] Unify virtual java providers

[Pascal Bach]

Hi guys
> It should be properly documented in the readme and the virtual packages should be the preferred way for users to depend on java/jre resp javac/jdk
>
> An alternative to the above would be:
>
> - virtual/jre
> - virtual/jre-native
> - virtual/jdk
> - virtual/jdk-native
>
> But this would deviate more from the current naming.
>
While testing the proposal above i ran into an issue.
When I add PROVIDES = "virtual/jre" to the java recipes and i want to install multiple java versions via update-alternative I can't do that because bitbake tells me:

"Multiple .bb files are due to be built which each provide virtual/jre:"

Now the question for me is if this two mechanism are not intended to work together or if I'm doing something wrong.

My idea would be to allow the user to select a default provider for "virtual/jre" but still allow to install an additional specific java provider if necessary.
Is this even possible?

Pascal