powerpc: Fix building in the old arch's boot directory for now

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index d948781..92751ca 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -152,13 +152,13 @@
 # All the instructions talk about "make bzImage".
 bzImage: zImage
 
-boot := arch/$(ARCH)/boot
+boot := arch/$(OLDARCH)/boot
 
 $(BOOT_TARGETS): vmlinux
-	$(Q)$(MAKE) $(build)=$(boot) $@
+	$(Q)$(MAKE) ARCH=$(OLDARCH) $(build)=$(boot) $@
 
 uImage: vmlinux
-	$(Q)$(MAKE) $(build)=$(boot)/images $(boot)/images/$@
+	$(Q)$(MAKE) ARCH=$(OLDARCH) $(build)=$(boot)/images $(boot)/images/$@
 
 define archhelp
   @echo '* zImage          - Compressed kernel image (arch/$(ARCH)/boot/images/zImage.*)'
@@ -171,7 +171,7 @@
 endef
 
 archclean:
-	$(Q)$(MAKE) $(clean)=arch/ppc/boot
+	$(Q)$(MAKE) $(clean)=$(boot)
 	# Temporary hack until we have migrated to asm-powerpc
 	$(Q)rm -rf arch/$(ARCH)/include