Add variable BUILT_REFERENCE_XML for callers to indicate dependencies for the
VTY reference.
Add script find_existing_path.sh to pick a given path from either builddir or
srcdir, whichever exists.
In Makefile.vty-reference.inc, use find_existing_path.sh to make the VTY
reference build rules work no matter whether the reference.xml is built in
builddir or committed in srcdir.
Change-Id: I613d692328050a036d05b49a436ab495fc2087ba
Neels Hofmeyr