1. STAGING_KERNEL_DIR was getting an invalid MLPREFIX. Fix this by forcing the expansion in multilib.conf.
2. Added multilib rule for 'perf'.
(From OE-Core rev: 71907775f601f0a48b995cc3277b61645d2e5068)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>