Image dependency generator cleanups

1) Remove whitespace from parsed filenames.
2) Make image file depend on dependency generator script.
3) Provide dummy rules for all dependencies. Without this change, if any
   dependency is renamed, make will try to build it but won't know how.
   With the dummy rules in place, make will execute an empty command to
   build any missing dependencies, which is fine if they've simply been
   renamed. Any real errors due to incorrect filenames in the current
   config files will be caught by the cbootimage invocation itself.

Signed-off-by: Stephen Warren <swarren@nvidia.com>