[OE-core] Use of multilib.conf taints hashes of -native recipes

[Mike Crowe]

We compile for a variety of machines, some of which require multilib.conf
and some that do not. I was surprised to discover when switching between
machines that the do_populate_sysroot_setscene tasks run for many -native
recipe. :(

It looks like this is because RECIPE_SYSROOT changes:

 Variable RECIPE_SYSROOT value changed from '${WORKDIR}/recipe-sysroot' to '${WORKDIR}/${MLPREFIX}recipe-sysroot'

This problem goes away if I add the following line to native.bbclass:

 RECIPE_SYSROOT = "${WORKDIR}/recipe-sysroot"

but this feels like I'm solving the problem in the wrong place. Would it be
better to modify multilib.conf to say something like:

 RECIPE_SYSROOT_class-target = "${WORKDIR}/${MLPREFIX}recipe-sysroot"

instead? (Although I'm rather worried that this will stop it actually
working.)

Thanks.

Mike.