qemuboot: Add QB_RNG variable

RNG passthru has been enabled on all qemu machines but its being added
to each one of them, with this patch its turned into QB variables which
defaults to host passthru, yet it can be overridden if needed via
machine or config metadata if needed.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>