shadow: Add nativesdk to BBCLASSEXTEND

shadow: Add nativesdk to BBCLASSEXTEND

This is a second in a series of patches to enable
offline rootfs creation from a package repository.

Some postinstall cmds are Yocto specific and needed to create a
rootfs with pre and post install hooks successfully run,
using only the toolchain tarball + a package repo.

End goal is to create a sandbox where users of a Yocto
based distribution can customize a rootfs from a package feed
with their package manager of choice.

With this patch, I can successfully create packagegroup-core-boot
with only the toolchain tarball(OPKG). More fixes for a few postinstall
hooks outside of packagegroup-core-boot will come next.

(From OE-Core rev: f90e1a45a042468e4e9a0fc91b57c6dba6f7adc9)

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