[PATCH] ppc64: reenable make install with defconfig

'make ARCH=ppc64 O=../O install' does not work with the defconfig.
CONFIG_PPC_BPA is part of it, but the BPA bootimage variable is wrong:

make[2]: *** No rule to make target `zImage', needed by `install'.  Stop.

Signed-off-by: Olaf Hering <olh@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>