Changes

buildsystem/gen_links: catch file not found errors

Tweak gen_links() to sequentially iterate over the files for which
symlinks shall be created, and verify that each symlink target exists.
If the target does not exist, abort with an error:

  ERROR in bsc/gen_links.sh: file not found
    File (from FILES):
      SDP_EncDec.cc
    DIR:
      ../deps/titan.Libraries.TCCUsefulFunctions/src

This is slightly slower than the parallel approach from before, but it
prevents adding subtle bugs that don't trigger every time so IMHO the
trade-off is worth it.

Change-Id: I6b0440dda7d5b37a4baa4b16f3979f7fb32b7b15