[POWERPC] Add gcc format warnings to zImage printf()

This patch adds the correct attributes to the zImage's versions of
printf to make gcc generate format string mismatch warnings.  It also
corrects several minor problems with format strings in the zImage thus
discovered.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>