Download source from ftp.rtems.org before the package's URL if
a release.
If a user adds a URL via the command line that is used then the
RTEMS release path then the package's URL.
A user can add --url=file://path/../morepath to have the RSB use
a local cache of source on their hard disk. This is useful if you
need to wipe the RSB and start again. Save away the 'sources' and
'patches' directories to a common directory and provide it with via
the --url option using the 'file://' prefix.
Closes#2482.