review: fix Makefile conditional

Author: bigbrett

Makefile

@@ -470,7 +470,7 @@ assemble_internal_flash.dd: FORCE
 		0 wolfboot.bin \
 		$$(($(WOLFBOOT_PARTITION_BOOT_ADDRESS) - $(ARCH_FLASH_OFFSET))) test-app/image_v1_signed.bin \
 		$$(($(WOLFBOOT_PARTITION_UPDATE_ADDRESS)-$(ARCH_FLASH_OFFSET))) /tmp/swap \
-		$(if $(DISABLE_BACKUP),,$$(($(WOLFBOOT_PARTITION_SWAP_ADDRESS)-$(ARCH_FLASH_OFFSET))) /tmp/swap) # swap unused with DISABLE_BACKUP
+		$(if $(filter 1,$(DISABLE_BACKUP)),,$$(($(WOLFBOOT_PARTITION_SWAP_ADDRESS)-$(ARCH_FLASH_OFFSET))) /tmp/swap) # omit swap partition if DISABLE_BACKUP=1
 
 internal_flash.dd: $(BINASSEMBLE) wolfboot.bin $(BOOT_IMG) $(PRIVATE_KEY) test-app/image_v1_signed.bin
 	@echo "\t[MERGE] internal_flash.dd"