Makefile: include defconfigs in sub-directories in list-defconfigs

Currently, list-defconfigs only lists the defconfigs that live
 live in the top-level configs/ directory. For the in-tree defconfigs
this is indeed the case, but it is possible to manage the configs in a
br2-external tree with sub-directories.

A few examples:
  - for a given board, a first defconfig is the full system, and a
    second is the rescue system;
  - for a given board, two defconfigs implement an A/B feature set;
  - a set of configurations targetting various famillies of systems each
    running on different hardware, sorted per familly.

Extend list-defconfigs to look for and report defconfigs in
sub-directories of the top-level configs/.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>
(cherry picked from commit 5009fd2436)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

Makefile

@@ -1218,7 +1218,7 @@ help:
 # $(2): br2-external name, empty for bundled
 define list-defconfigs
 	@first=true; \
-	for defconfig in $(1)/configs/*_defconfig; do \
+	for defconfig in $$(find $(1)/configs -name '*_defconfig' |sort); do \
 		[ -f "$${defconfig}" ] || continue; \
 		if $${first}; then \
 			if [ "$(2)" ]; then \
@@ -1228,7 +1228,7 @@ define list-defconfigs
 			fi; \
 			first=false; \
 		fi; \
-		defconfig="$${defconfig##*/}"; \
+		defconfig="$${defconfig#$(1)/configs/}"; \
 		printf "  %-35s - Build for %s\n" "$${defconfig}" "$${defconfig%_defconfig}"; \
 	done; \
 	$${first} || printf "\n"