Revert "Makefile: ensure CONF_USR_DIR honours the libdir config"

Revert "Makefile: ensure CONF_USR_DIR honours the libdir config"

LIBDIR in Debian and derivatives is not /usr/lib/, it's
/usr/lib/<architecture triplet>/, which is different, and it's the
wrong location where to install architecture-independent default
configuration files, which should always go to /usr/lib/ instead.
Installing these files to the per-architecture directory is not
the right thing, hence revert the change.

This reverts commit 946753a4459bd035132a27bb2eb87529c1979b90.

Signed-off-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>