[POWERPC] boot: rebuild when wrapper changes

Since there is magic defined per platform in the wrapper script, the
zImage targets should depend on it.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>