[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>

arch/ppc64/Makefile

@@ -108,7 +108,7 @@ $(boottargets-y): vmlinux
 bootimage-$(CONFIG_PPC_PSERIES) := $(boot)/zImage
 bootimage-$(CONFIG_PPC_PMAC) := vmlinux
 bootimage-$(CONFIG_PPC_MAPLE) := $(boot)/zImage
-bootimage-$(CONFIG_PPC_BPA) := zImage
+bootimage-$(CONFIG_PPC_BPA) := $(boot)/zImage
 bootimage-$(CONFIG_PPC_ISERIES) := vmlinux
 BOOTIMAGE := $(bootimage-y)
 install: vmlinux