Cache bnd repo in cnf/cache/

Caching in the default location of ~/.bnd/cache caused some build
failures because the bnd repo changes for each bnd build and that upset
the repo logic occasionally.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Use bnd_repourl in gradle.properties as the location of the bnd repo build

This allows the repo url to be externally set in the CI build with the
gradle command line option -Pbnd_repourl=<url>.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>