[POWERPC] boot: Use a common zImage rule

Before the plethora of platforms gets any worse, establish a common
rule to invoke the wrapper for any platform.  Add arguments to
the rule for initrd, dts, dtb, etc.   Show example usage with initrd.

Create default rules for zImage, and zImage.initrd.  initrd targets
depend on the ramdisk file.

Don't consider targets for zImage.initrd that are targets for zImage.
This means uImage is no longer considered a target for zImage.initrd.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>