iterate_dir: Include symlinks to dirs also (#97)

Given:
`for root, subdirs, filelist in os.walk(dir):`

.. symlinks to directories are only returned in `subdirs`. So, they need
to handled explicitly.

This meant that when bockbuild tried to zip up the contents of mono and
msbuild to move to the staging directory, then such symlinks were not
picked up thus breaking the build.

(cherry picked from commit 5d818c1)

bockbuild/util/util.py

@@ -548,6 +548,12 @@ def iterate_dir(dir, with_links=False, with_dirs=False, summary=False):
         dirs = dirs + 1
         if with_dirs:
             yield root
+        if with_dirs and with_links:
+            for subdir in subdirs:
+                path = os.path.join(root, subdir)
+                if os.path.islink(path):
+                    links = links + 1
+                    yield path
         for file in filelist:
             path = os.path.join(root, file)
             if os.path.islink(path):