classes/multilib: prevent multilib extension of nativesdk recipes

classes/multilib: prevent multilib extension of nativesdk recipes

It isn't supported to mix multilib and nativesdk in the same target, so
explicitly skip multilib processing if nativesdk is inherited. As a
bonus this fixes a bunch of related "missing file" warnings from the
file checksum code during parsing because BPN was not correctly stripped
for these targets.

Second half of the fix for [YOCTO #3146].

(From OE-Core rev: d9a1eb5054d487affb94431374a9cb1a735e2122)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>