gradle: build the projects with Gradle (v6)

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
cnf: gradle: we need an up-to-date bnd for the build

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
cnf: gradle: we have the latest repoindex in the gradle dependencies

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
bndoc: gradle: do not include this project in the build

Like was done for the Ant build

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
bndlib.tests: gradle: fix the test directory setup

So that it can be built by the template

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
dist: gradle: add a dist task that releases everything and indexes the release repo

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>
WIP doc