So that a single machine can build multiple architectures for the
uninative-tarball respect SDK_ARCH instead of BUILD_ARCH.
This means a x86-64 host can build a i686 uninative-tarball by setting
SDKMACHINE=i686.
(From OE-Core rev: 11b0e7e1cb29fd1fbe06bdb5606a55b92ecdcc89)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>