Makefile: generate-files: fix check for empty TMP_OUT

commit c655b7d added a check to make sure
the TMP_OUT variable was not set to an empty value, as such a situation would
perform an `rm -rf /**` during cleanup.

However, it was a bit too eager, because Makefile conditionals (`ifeq`) are
evaluated when parsing the Makefile, which happens _before_ the make target
is executed.

As a result `$@_TMP_OUT` was always empty when the `ifeq` was evaluated,
making it not possible to execute the `generate-files` target.

This patch changes the check to use a shell command to evaluate if the var
is set to an empty value.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 25c9e6e)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Makefile

@@ -250,9 +250,10 @@ swagger-docs: ## preview the API documentation
 .PHONY: generate-files
 generate-files:
 	$(eval $@_TMP_OUT := $(shell mktemp -d -t moby-output.XXXXXXXXXX))
-	ifeq ($($@_TMP_OUT),)
-		$(error Could not create temp directory.)
-	endif
+	@if [ -z "$($@_TMP_OUT)" ]; then \
+		echo "Temp dir is not set"; \
+		exit 1; \
+	fi
 	$(BUILD_CMD) --target "update" \
 		--output "type=local,dest=$($@_TMP_OUT)" \
 		--file "./hack/dockerfiles/generate-files.Dockerfile" .