RFR: Build: Update to JDK 15.0.2

[Andrea Cosentino]

On Wed, 20 Jan 2021 17:40:45 GMT, Andrea Cosentino <github.com+5106647+oscerd at openjdk.org> wrote:

> Hello all,
> 
> Just a little PR for upgrading to JDK 15.0.2.
> 
> make images passed fine and make test too.
> 
> Thank you

I believe I'm covered under the blanket Red Hat agreement

-------------

PR: https://git.openjdk.java.net/skara/pull/989