This patch add support for a system control block. It is supposed to
act as helper for the emulated program. E.g. shutting down the VM or
printing test results. This model is intended for testing purposes only and
doesn't fit to any real hardware. Therefore, it is not added to any board
by default. Instead a user has to add it explicitly with the '-device'
commandline parameter.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>