Makefile: error out printvars target when VARS is empty or unset

printvars returns nothing when VARS is not passed or empty. This is done
on purpose, see commit fd5bd12379dc ("Makefile: printvars: don't print
anything when VARS is not set").

An error message making explicit what is required from the user in order
to use printvars is however better than silently doing nothing.

This adds a check for a non-empty VARS variable.

Cc: Quentin Schulz <foss+buildroot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit ec82347cdee190a5fa451b83cdb3727c78cda315)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -1059,6 +1059,10 @@ endif
 # to workaround a bug in make 4.3; see https://savannah.gnu.org/bugs/?59093
 .PHONY: printvars
 printvars:
+ifndef VARS
+	@echo "Please pass a non-empty VARS to 'make printvars'"
+	@exit 1
+endif
 	@:
 	$(foreach V, \
 		$(sort $(foreach X, $(.VARIABLES), $(filter $(VARS),$(X)))), \