hw/arm/boot: allow using a command line specified dtb without a kernel

When kernel and device tree are specified in the QEMU commandline, then
this device tree may be modified e.g. to add virtio_mmio devices.
With a bootloader e.g. on a flash device these extra devices are not
available.
With this change, the device tree can be specified at the QEMU commandline.
The modified device tree made available to the bootloader with the same
mechanism already supported by device trees fully generated by QEMU.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Message-id: 1473520054-402-1-git-send-email-m.olbrich@pengutronix.de
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>