package(rootfs)_ipk.bbclass: support multilib in opkg backend.
Support install multiple multilib in opkg backend.
The installation is done in 3 phases.
Phase 1: install normal packages to IMAGE_ROOTFS.
Phase 2: install multilib packages under MULTILIB_TEMP_ROOTFS. Packages
belongs to the same multilib arch would be installed to a unique
directory.
Phase 3: check file confliction between IMAGE_ROOTFS and
MULTILIB_TEMP_ROOTFS, install multilib packages to IMAGE_ROOTFS only if
the sanity check passed.
