nativesdk-packagegroup-sdk-host: Adding nativesdk-postinst-intercept to SDK

nativesdk-packagegroup-sdk-host: Adding nativesdk-postinst-intercept to SDK

Adding ability to use postinstalls intercepts in the nativesdk env, and
making sure the correlate between repo + SDK.

This to enable rootfs generation from a package repository using only a
package repository and the toolchain tarball.

See https://github.com/nysan/rootfs-sandbox for examples.

Signed-off-by: David Nyström <david.nystrom@enea.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>