[POWERPC] boot: clean rule fixes

Now that obj-boot is in targets, we can remove (twice) it from clean-files.
zImage.initrd was missing, move zImage nearer where its used, and place
zImage.initrd next to it.  Remove non-ported zImage.sandpoint, and add
auto-generation of unconfigured zImage.initrd* like image-.

Signed-off-by: Milton Miller <miltonm@bga.com>
--
Testing: did a few builds and cleans
Signed-off-by: Paul Mackerras <paulus@samba.org>