When multilib is enabled, STAGING_DATADIR is expanded with MLPREFIX. But
dependencies of images such as syslinux is still populated to nonarch
STAGING_DATADIR.
Search nonarch STAGING_DATADIR to fix the error.
Signed-off-by: Kai Kang <kai.kang@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>