Makefile: fix list-defconfigs for br2-external with weird descriptions

When a br2-external tree description contains an odd number of single
quotes, list-defconfigs will fail with a shell error message:

    /bin/sh: -c: line 1: unexpected EOF while looking for matching `"'
    make[1]: *** [Makefile:1243: list-defconfigs] Error 2

Whoever wrote that code will have to write this sentence 100 times
on the blackboard:

     Never, ever pass user-provided data in a printf format.

(see commit 49117c102803 (core: support description for br2-external
trees); dang, I knew it... And I should have known better when I wrote
that code, damit...)

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

Makefile

@@ -1223,7 +1223,7 @@ define list-defconfigs
 		[ -f "$${defconfig}" ] || continue; \
 		if $${first}; then \
 			if [ "$(2)" ]; then \
-				printf 'External configs in "$(call qstrip,$(2))":\n'; \
+				printf 'External configs in "%s":\n' "$(call qstrip,$(2))"; \
 			else \
 				printf "Built-in configs:\n"; \
 			fi; \