gradle: allow configuration of the name on a repository index

Syntax:
project.ext.gradleBuild_indexDirectories = \
  "<directory>;<name>,<directory>;<name>,..."

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
dist: set the name 'bnd' on the repository indexes

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