Adjust vty_reference_combine.sh, so we can change the output dir from
"generated" to a subdir inside "generated" when building anything but
the first VTY reference.
Add a generic pattern to Makefile.vty-reference for additional VTY
references, that uses slightly different paths compared to the first VTY
reference, in order to avoid collisions.
Related: OS#4292
Change-Id: I4dea3e07b88175b2a88e577129360af7ec5f87e1