eglibc-locale: Specially handle FILES_eglibc-gconv for multilib

eglibc-locale: Specially handle FILES_eglibc-gconv for multilib

In multilib support, it iterate values in PACKAGES and then extend name
for variables like "FILES_xxx", "SUMMARY_xxx", etc.

However eglibc-gconv is dynamically put in PACKAGES by
package_do_split_gconv function. Therefore the name will not be
extended automatically.

Specially handle the FILES variable for eglibc-gconv to fix the issue
of missing "lib32-eglibc-gconv" issue in doing multilib do_rootfs.

Also when set PACKAGES, add the MLPREFIX.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>