Minor cleanup: remove superfluous semicolon.
Improve build feature handling with HAIKU_NO_DOWNLOADS=1

* Adjust the respective rules such that with disabled downloads, only
  packages already available in the downloads folder will be considered
  as available build features.

This way, the build system will not for instance try to build
<kdebug>qrencode after a bootstrap, as that package is not yet
available.
Add variable noDownloads to build_haiku_image.

* If noDownloads is set, show an error when a download is requested.