5/llvm: Add LLVM as a package for RTEMS.

- Add '%source download <source>' to only download the source
  and do not unpack and prep. This can used when a package
  internally needs another source package.

- Install the staging root only if it is present. A package
  may internally build another package that is not staged as it
  is not suitable for installing.

Updates #3250
Updatew #3797

source-builder/sb/freebsd.py

@@ -63,6 +63,7 @@ def load():
         '_var':             ('dir',     'optional', '/usr/local/var'),
         '__bash':           ('exe',     'optional', '/usr/local/bin/bash'),
         '__bison':          ('exe',     'required', '/usr/local/bin/bison'),
+        '__cmake':          ('exe',     'optional', '/usr/local/bin/cmake'),
         '__git':            ('exe',     'required', '/usr/local/bin/git'),
         '__svn':            ('exe',     'optional', '/usr/local/bin/svn'),
         '__unzip':          ('exe',     'optional', '/usr/local/bin/unzip'),