unified: dissociate system workqueue from common workqueue module

Making a reference to the common work queue code should not necessarily
drag in the system workqueue, since it is possible to use a workqueue
that is not the system workqueue. This is done by moving the system
workqueue into its own code module.

Moving the system workqueue to its own code module allows removing the
NANO_WORKQUEUE and SYSTEM_WORKQUEUE kconfig options, and compiling the
common workqueue code and system workqueue all the time. They are only
linked in the final image if a reference to them exist, same as the
other kernel modules.

Change-Id: I6f48d2542bda24f4702e7c2e317818dd082b3c11
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>