allarch: only enable allarch when multilib is not used

allarch: only enable allarch when multilib is not used

Some allarch packages rdepends non-allarch packages. when multilib is
used, it doesn't expand the dependency chain correctly, e.g.

core-image-sato -> ca-certificates(allarch) -> openssl

we expect dependency chain for lib32-core-image-sato:

lib32-core-image-sato -> ca-certificates(allarch) -> lib32-openssl

it should install lib32-openssl for ca-certificates but openssl is still
wrongly required.

Only enable allarch when multilib is not used to fix the issue.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>