Hardcoding -nativesdk as the sdk package architecture is inflexible. We may have
multiple different target OS and we need a way to be able to separate them. Turning
this into a configurable value allows the flexibility we need to build different
SDKMACHINEs with different OS targets.
The commit should have no behaviour change, just makes things more configurable.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
