[POWERPC] cell: fix default zImage build target

Change CONFIG_PPC_CELL to CONFIG_PPC_IBM_CELL_BLADE in the powerpc boot
makefile.

CONFIG_PPC_CELL is used to build the generic cell processor support, and
is not an indication of platform.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>