Prevent symlinks to be added to the tree

Symlinks cannot be allowed because they are not supported by distutils.
Adding them results into package build failures.

This patch add a check that verifies that no symlinks can slip in.

Closes-bug: #1404605

Change-Id: I0d308127081bb2fa4ff7d7e0ed2f1b6e915163c9
Merge "Prevent symlinks to be added to the tree"