image.bbclass: Added variables for multilib support.

image.bbclass: Added variables for multilib support.

1. Added MULTILIB_PACKAGE_INSTALL for multilib instances of packages to
be installed in the rootfs.

2. MULTILIBRE_ALLOW_REP contains the regular expression to match the
files allow to be replaced by the conflicting files.

3. MULTILIBRE_FORCE_SAME contains the regular expression to match the
files allow to be replaced only if the conflicting files are identical.

4. Added shell function multilib_sanity_check() to check whether the
overwring for multilib situation is allowed.

Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>