ipk: use OPKGLIBDIR in all places

* it's not recommended to change this value, because it breaks upgrade path on target
  (all old u-a alternatives are forgot in old OPKGLIBDIR value
* but make it consistent, so if someone really want to change that, then
  setting OPKGLIBDIR_distro in distro.conf would be enough
* without this there were at least 4 places to change:
  rootfs_ipk: opkglibdir variable (notice that I've removed /opkg from it to correspond
              with EXTRA_OECONF option used in opkg recipes
  package_ipk: ${target_rootfs}${localstatedir}/lib/opkg/ hardcoded in
               package_install_internal_ipk
  opkg-collateral: value in lists file
  opkg: EXTRA_OECONF for all 3 classes, FILES_libopkg, do_install
* validated with buildhistory that without OPKGLIBDIR explicitly set the
  output is the same and that after setting
  OPKGLIBDIR_forcevariable := "${libdir}"
  everything including empty directory from package_ipk is moved to
  libdir

(From OE-Core rev: cf0aa9c4fdae8855803e96b1922d54a2431795d3)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/package_ipk.bbclass

@@ -12,6 +12,8 @@ OPKGBUILDCMD ??= "opkg-build"
 
 OPKG_ARGS = "-f $INSTALL_CONF_IPK -o $INSTALL_ROOTFS_IPK --force_postinstall --prefer-arch-to-version"
 
+OPKGLIBDIR = "${localstatedir}/lib"
+
 python package_ipk_fn () {
     d.setVar('PKGFN', d.getVar('PKG'))
 }
@@ -138,7 +140,7 @@ package_install_internal_ipk() {
 	local package_to_install="${INSTALL_PACKAGES_NORMAL_IPK}"
 	local package_multilib="${INSTALL_PACKAGES_MULTILIB_IPK}"
 
-	mkdir -p ${target_rootfs}${localstatedir}/lib/opkg/
+	mkdir -p ${target_rootfs}${OPKGLIBDIR}/opkg
 
 	local ipkg_args="${OPKG_ARGS}"