package(_ipk).bbclass: opkg using ALL_MULTILIB_PACKAGE_ARCHS
[YOCTO #1345]
The new variable ALL_MULTILIB_PACKAGE_ARCHS contains all the values of
PACKAGE_ARCHS for each multilib variants. The opkg backend now uses this
new value insteald of the PACKAGE_ARCHS to update the opkg indexes and
to generate the opkg configuration files. This allows the normal
packages and multilib packages may be installed into the same rootfs.
