Whitespace cleanup in root and 'build' folder.

* Removed trailing whitespace and empty lines at top of files.
Create packages_build folders per build type.

* The content of the preprocessed package-info files and the package
  contents depend on the build type, so we use a different folder for
  each build type.
Add new configuration option --no-downloads.

* If --no-downloads has been given, Haiku will be built without
  trying to download anything, all required packages need to be put
  into the download folder manually (the build will stop on missing
  packages).
* As the required HaikuPorts repository can't be downloaded in this
  mode, a local repository is created during the build, which only
  contains the packages available in the downloads folder.

This is useful for building Haiku completely from source.