Now that RHELBLD-12538 is complete, we need a similar setup on the CentOS side to be able to build OpenJDK there.

The current RHEL setup is:

• Two build roots, java-openjdk-rhel-8-build (inheriting from 8.8.0-z) and java-openjdk-rhel-9-build (inheriting from 9.2.0-z)
• Packages built in these buildroots only appear in that buildroot
• The java-<x>-openjdk-portable packages are built in java-openjdk-rhel-8-build only
• The java-openjdk-rhel-9-build also inherits from java-openjdk-rhel-8-build
• The java-<x>-openjdk packages are built in both java-openjdk-rhel-8 and java-openjdk-rhel-9
• The java-<x>-openjdk packages are tagged into rhel-8.8.0-z-gate and rhel-9.2.0-z-gate as appropriate to make them visible in the rest of RHEL
• The java-<x>-openjdk-portable are never made visible to the rest of RHEL and are not shipped

The current java-<x>openjdk packages in c9s expect java<x>-openjdk-portable to be available to build, so we need:

• A branch per java-<x>-openjdk package where the portable work can be done (this is openjdk-portable-rhel-8 in the RHEL ones)
• A buildroot where java-<x>-openjdk-portable can be built, isolated from the rest of CentOS
• A means to tag java-<x>-openjdk builds from the isolated buildroot into CentOS

See the RHEL subtasks for more details of the setup there.