Create specific config option for "platform-bus"

Create specific config option for "platform-bus"

Currently the "platform-bus" device is included for all softmmu builds.
This bridge is intended for use on any platforms that require dynamic
creation of sysbus devices.  However, at present it is used only for the
PPC E500 target, with plans for the ARM "virt" target in the immediate
future.

To avoid a not-very-useful entry appearing in "qemu -device ?" output on
other targets, this patch makes a specific config option for platform-bus
and enables it (for now) only on ppc configurations which include E500
and on ARM (which always includes the "virt" target).

Signed-off-by: David Gibson <[email protected]>
Message-Id: <1425017077[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>