packagegroup-core-standalone-sdk-target: Add qemuwrapper-cross to target sysroot

Some postinstall scripts use the qemuwrapper script, so to be able to
offline install these packages outside of the bitbake environment, this script
needs to be exposed also in the 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>