Makefile: enhance reporting error about empty VARS

Use $(error) to simplify the code (drop "exit 1") and sned the message
to stderr.

Reported-by: David Laight <David.Laight@ACULAB.COM>
Reported-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Quentin Schulz <quentin.schulz@theobroma-systems.com>
(cherry picked from commit 466d65d73e38545252528a81c31632ba316c77fa)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -1060,8 +1060,7 @@ endif
 .PHONY: printvars
 printvars:
 ifndef VARS
-	@echo "Please pass a non-empty VARS to 'make printvars'"
-	@exit 1
+	$(error Please pass a non-empty VARS to 'make printvars')
 endif
 	@:
 	$(foreach V, \