multilib.bbclass: set rpm file color for 32-bit multilib image

The default rpm file color is 2 that Elf64 is preferred according to
OE-Core commit 56fa7449. For 32-bit
multilib image such as lib32-core-image-sato, Elf32 should be preferred.
Set it in multilib.bbclass.

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