Makefile: fix show-vars for good this time

Commit 5c54c3ef3d (Makefile: workaround make 4.3 issue for 'printvars
and 'show-vars') did not fully fix the show-vars case, which still
segfaults.

Overall, show-vars generates a JSON blurb. That is supposed to be
machine-readable, so we do not care that the variables are sorted, so
we get rid of it to (slightly) simplify the code.

Then, we currently iterate twice on the list of variables: the first one
to filter-out the 'internal' variables, and the second one to filter
only the variables matching the pattern. We can do away by iterating
only once, and applying both filters at once.

Since we now have an 'and' condition, we can take advantage of it: when
none of the items in $(and) are empty, $(and) evaluates to the last
item, while it evaluates to empty if any of the items is empty. So we
can coalesce the $(if) and $(and) together: $(if $(and a,b),c) is
equivalent to: $(and a,b,c) ; this gains us one parentheses depth.

Finally, the cause for the segfault is an overly-long call to $(info).
Reducing that is not easy: we want to call clean-json on the whole of
the JSON blurb, so we can't emit the individual variables one by one, or
the trailing comma would not be trimmed away.

So, we go crazy: we just output each word from clean-json with $(info).

We can do that, because mk-json-str transforms all spaces in a string
to an escaped UTF-8 sequence, so we will never have spaces in values;
the keys are the variables, so they won't have spaces either; spaces in
the rest of the JSON blurb are totally optional, so we don't care how
many there are. We know there are spaces, because we explicitly
introduce some (after "expanded" or "raw", for example), so we should
never hit a too-big word for $(info) to print.

Thanks to Henri for the suggestion to push $(info) further inside the
macro.

Reported-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Roosen Henri <Henri.Roosen@ginzinger.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Makefile

@@ -1073,17 +1073,24 @@ printvars:
 show-vars: VARS?=%
 show-vars:
 	@:
-	$(info $(call clean-json, { \
+	$(foreach i, \
+		$(call clean-json, { \
 			$(foreach V, \
-				$(sort $(foreach X, $(.VARIABLES), $(filter $(VARS),$(X)))), \
+				$(.VARIABLES), \
-				$(if $(filter-out environment% default automatic, $(origin $V)), \
+				$(and $(filter $(VARS),$(V)) \
+					, \
+					$(filter-out environment% default automatic, $(origin $V)) \
+					, \
 					"$V": { \
 						"expanded": $(call mk-json-str,$($V))$(comma) \
 						"raw": $(call mk-json-str,$(value $V)) \
 					}$(comma) \
 				) \
 			) \
-	} ))
+		} ) \
+		, \
+		$(info $(i)) \
+	)
 
 .PHONY: clean
 clean: