The n32 MIPS rust SDK doesn't quite build (libstd-rs fails with an llvm
register issue). Disable it for now, someone with interest in having it working
can fix if/as/when.
(From OE-Core rev: aad10f24eea5a6b54de17138ab5c0f4ed1355a8f)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>