This adds support for building a barebox image that boots with the
Linux ARM Kernel booting convention. Support for this image can be
enabled in Kconfig. It picks up a device tree passed in r2. This new
image helps for example with qemu. It can be started with:

qemu-system-aarch64 -m 2G -M virt -kernel images/barebox-dt-2nd.img -cpu cortex-a57 -serial stdio

or:

qemu-system-arm -m 1G -M sabrelite -kernel images/barebox-dt-2nd.img -nographic -dtb arch/arm/dts/imx6q-sabrelite.dtb

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>