Install rustc and cargo into SYSROOT

[nastevens]

Bringing over this comment from #34 or else I'll never remember it:

BTW, I've noticed that this layer does not install rustc and cargo into ${SYSROOTS}/x86_64-linux on Yocto 2.1(jethro).
Is this behavior intended?
I guess this behavior prevents mixing binaries for different architectures.