gradle: build the projects with Gradle (v6)

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
cnf: gradle: add class cycle checks

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
build: gradle: add p2 generation

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
root: gradle: also distclean the releaserepo

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: add a (temporary) script to fully exercise the build

I keep having to look up all the targets...

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>