Export constant attribute for the attributes only in the device context

[martonmiklos]

EAGLE claiming about the exported files because the constant field is set for the elements in the board/elements attributes.

eagle.dtd has the solution

          <!-- display: Only in <element> or <instance> context -->
          <!-- constant:Only in <device> context -->

To properly solve this issue kode #35 needs to be implmented (implement class override) and then it needs to be addressed.