libtool: change the default AR_FLAGS from "cru" to "cr"

Backport patch to fix warning:
`u' modifier ignored since `D' is the default (see `U')

Signed-off-by: Li Wang <li.wang@windriver.com>
Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>