[POWERPC] When appropriate, wrap device tree with zImage

There are 2 config options that indicate whether the platform being built
has a device tree source file associated with it.  Namely,
CONFIG_WANT_DEVICE_TREE and CONFIG_DEVICE_TREE.  When CONFIG_WANT_DEVICE_TREE
is 'y' and CONFIG_DEVICE_TREE isn't an empty string, automatically wrap
the specified device tree with the zImage being built.

To achieve this, the 'dts' variable will only be set when the conditions
above are true.  The changes to the zImage.initrd.% and zImage.% rules
cause the device tree to be wrapped when 'dts' is set; otherwise, they
will work as they previosly did (i.e., build a zImage with no device tree).

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>