postinst-intercept: New recipe to include postinstall intercepts in nativesdk

postinst-intercept: New recipe to include postinstall intercepts in nativesdk

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>