gradle: update documentation

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: clear the sourceSets output directories in the clean task

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: stronger asserts on the non bnd project pre hook

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: properly set the licenses

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