PPC32: Fix the rule for using config files from arch/ppc/configs/

To accommodate the change to scripts/Configure which made it look in
/boot/config-`uname -r` before looking at arch/$(ARCH)/defconfig, change the
rule for using a config from arch/ppc/configs to copy to .config instead of
arch/$(ARCH)/defconfig

arch/ppc/Makefile

@@ -108,7 +108,7 @@ $(BOOT_TARGETS): vmlinux
 
 %_config: arch/ppc/configs/%_defconfig
 	rm -f .config arch/ppc/defconfig
-	cp -f arch/ppc/configs/$(@:config=defconfig) arch/ppc/defconfig
+	cp -f arch/ppc/configs/$(@:config=defconfig) .config
 
 archclean:
 	rm -f arch/ppc/kernel/{mk_defs,asm-offsets.h,find_name}