Makefile: Pass DOCKER_ARCH_TAG to makeiso.sh script

We need to pass the target architecture to makeiso.sh script in order to
run the corresponding pkg/mkimage-iso-efi container.

Signed-off-by: Renê de Souza Pinto <rene@renesp.com.br>
(cherry picked from commit c5b3519)

Makefile

@@ -789,7 +789,7 @@ $(INSTALLER).raw: $(INSTALLER_IMG) $(EFI_PART) $(BOOT_PART) $(CONFIG_IMG) $(BSP_
 	$(QUIET): $@: Succeeded
 
 $(INSTALLER).iso: $(INSTALLER_TAR) $(BSP_IMX_PART) | $(INSTALLER)
-	./tools/makeiso.sh $< $@ installer
+	DOCKER_ARCH_TAG=$(DOCKER_ARCH_TAG) ./tools/makeiso.sh $< $@ installer
 	$(QUIET): $@: Succeeded
 
 $(INSTALLER).net: $(INSTALLER).iso $(EFI_PART) $(INITRD_IMG) $(CONFIG_IMG) | $(INSTALLER)