gradle: Fix comment

We don't use cnf property in master.gradle.
gralde: Put bnd project in a variable for use by customizers

Customizers may want to perform customizations in the master.gradle
script, so we put the bnd project in a variable since the may need to
use it.
gradle: Handle cnf project not being a bnd project

The bndtools workspace template creates the cnf project with no bnd.bnd
file.
gradle: Allow bnd_build property to be empty

This change allows there to be no "master" project in the workspace.
Without a "master" project whose dependencies will seed the set of
projects to build, all non-private folders (names don't start with ".")
will be added to the set of gradle projects.
gradle: bnd only supports a single src folder

More correctly model bnd in the gradle plugin. Use Project.getSrc()
instead of Project.getSourcePath() to get the single source folder.
See issue #520.