Makefile: printvars: don't print anything when VARS is not set

Using 'make printvars' for printing all variables is not very useful.
E.g. all macros will output some bogus value. In addition, the same can
be achieved with 'make -p'.

We can simply remove the condition on $(VARS). If VARS is not set, the
filter expression will be empty which matches nothing, so nothing is
printed.

Note that the old behaviour can still be achieved with:
make printvars VARS=%

Update the 'make help' text to match the new behaviour.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Makefile

@@ -1031,7 +1031,7 @@ $(BUILD_DIR)/.br2-external.in: $(BUILD_DIR)
 printvars:
 	@:
 	$(foreach V, \
-		$(sort $(if $(VARS),$(filter $(VARS),$(.VARIABLES)),$(.VARIABLES))), \
+		$(sort $(filter $(VARS),$(.VARIABLES))), \
 		$(if $(filter-out environment% default automatic, \
 				$(origin $V)), \
 		$(if $(QUOTED_VARS),\
@@ -1124,7 +1124,7 @@ help:
 	@echo '  source                 - download all sources needed for offline-build'
 	@echo '  external-deps          - list external packages used'
 	@echo '  legal-info             - generate info about license compliance'
-	@echo '  printvars              - dump all the internal variables'
+	@echo '  printvars              - dump internal variables selected with VARS=...'
 	@echo
 	@echo '  make V=0|1             - 0 => quiet build (default), 1 => verbose build'
 	@echo '  make O=dir             - Locate all output files in "dir", including .config'