Makefile: use /usr/share/iproute2 for config files

Makefile: use /usr/share/iproute2 for config files

According to FHS:

"/usr/lib includes object files and libraries. On some systems, it may
also include internal binaries that are not intended to be executed
directly by users or shell scripts."

A better directory to store config files is /usr/share:

"The /usr/share hierarchy is for all read-only architecture independent
data files.

This hierarchy is intended to be shareable among all architecture
platforms of a given OS; thus, for example, a site with i386, Alpha, and
PPC platforms might maintain a single /usr/share directory that is
centrally-mounted."

Accordingly, move configuration files to $(DATADIR)/iproute2.

Fixes: 946753a4459b ("Makefile: ensure CONF_USR_DIR honours the libdir config")
Reported-by: Luca Boccassi <luca.boccassi@gmail.com>
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Acked-by: Luca Boccassi <bluca@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>