java/openjdk25: Add jre and headless flavors

Add FLAVORS to allow building jre and headless variants from the same
port. For backwards compatibility, it is also possible to build the jre
version by setting the BUILD_JRE env var.

Also reorganize the Makefile to make portclippy happy

Reviewed by: fuz, jrm
Approved by: fuz (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53450