[POWERPC] Add Makefile rules to wrap dts file in zImage

Add 'zImage.dts' and 'zImage.dts_initrd' build rules that automatically
compile and wrap a dts file from arch/powerpc/boot/dts into the zImage file.
The resulting zImage will be arch/powerpc/boot/zImage.dts.<platform> and
arch/powerpc/boot/zImage.dts_initrd.<platform>, respectively.

Having separate rules allows the user to choose whether to include a device
tree--and which device tree--at build time.  This is useful when one Makefile
target builds a zImage that runs on several platforms except for differing
device trees.  By just setting CONFIG_DEVICE_TREE and running "make zImage.dts"
the exact zImage you want is built without Makefile bloat or manually running
the wrapper script.

The dts file is expected to be arch/powerpc/boot/dts/$(CONFIG_DEVICE_TREE)

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>