target-arm: Add vexpress machine secure property

Add "secure" Vexpress machine specific property to allow override of the
default secure state configuration.  By default, when using the QEMU
-kernel command line argument, Vexpress machines boot into NS/SVC.  When using
the QEMU -bios command line argument, Vexpress machines boot into S/SVC.

The secure state can be changed from the default specifying the secure
state as a machine property.  For example, the below command line would disable
security extensions on a -kernel Linux boot:

    aarch64-softmmu/qemu-system-aarch64
        -machine type=vexpress-a15,secure=off
        -kernel ...

Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1418684992-8996-5-git-send-email-greg.bellows@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
