util-linux: add setpriv utility

util-linux: add setpriv utility

Enable the setpriv utility for targets only. It will be used in
the run-ptest script for bash and perhaps other packages where
the ptest code is expected to run as a user.

setpriv uses libcap-ng which doesn't build natively so disable
it for native* builds. Also, busybox has a setpriv implementation
so ensure that setpriv adheres to the alternatives scheme.

Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>