bitbake.conf: default SDKMACHINE to the build host architecture

Currently there's a limitation that Extensible SDKs can only be build on a
machine with the same architecture as the SDK.

However, SDKMACHINE defaults to x86-64, which means eSDKs cannot be built
out of the box on arm64 systems.

By defaulting SDKMACHINE to ${BUILD_ARCH} we generate SDKs for the build
host, so they'll always work out of the box.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>