kbuild: show defconfig subdirs in make help

PowerPC will start moving board defconfigs into subarch-specific
subdirs soon.  "make help" currently does not look in subdirs to
find the defconfigs to show.  This is partially a good thing,
since there are way too many defconfigs for one list.

This patch makes the main "make help" display something like

  help-40x         - Show 40x-specific targets
  help-44x         - Show 44x-specific targets
  help-boards      - Show all of the above

and wires up stuff so those new help-* commands actually work.

[sam: fixed it up to display x86 defconfigs too]
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1174,8 +1174,10 @@ rpm: include/config/kernel.release FORCE
 # Brief documentation of the typical targets used
 # ---------------------------------------------------------------------------
 
-boards := $(wildcard $(srctree)/arch/$(ARCH)/configs/*_defconfig)
+boards := $(wildcard $(srctree)/arch/$(SRCARCH)/configs/*_defconfig)
 boards := $(notdir $(boards))
+board-dirs := $(dir $(wildcard $(srctree)/arch/$(SRCARCH)/configs/*/*_defconfig))
+board-dirs := $(sort $(notdir $(board-dirs:/=)))
 
 help:
 	@echo  'Cleaning targets:'
@@ -1229,6 +1231,11 @@ help:
 		$(foreach b, $(boards), \
 		printf "  %-24s - Build for %s\\n" $(b) $(subst _defconfig,,$(b));) \
 		echo '')
+	@$(if $(board-dirs), \
+		$(foreach b, $(board-dirs), \
+		printf "  %-16s - Show %s-specific targets\\n" help-$(b) $(b);) \
+		printf "  %-16s - Show all of the above\\n" help-boards; \
+		echo '')
 
 	@echo  '  make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build'
 	@echo  '  make V=2   [targets] 2 => give reason for rebuild of target'
@@ -1240,6 +1247,20 @@ help:
 	@echo  'For further info see the ./README file'
 
 
+help-board-dirs := $(addprefix help-,$(board-dirs))
+
+help-boards: $(help-board-dirs)
+
+boards-per-dir = $(notdir $(wildcard $(srctree)/arch/$(SRCARCH)/configs/$*/*_defconfig))
+
+$(help-board-dirs): help-%:
+	@echo  'Architecture specific targets ($(SRCARCH) $*):'
+	@$(if $(boards-per-dir), \
+		$(foreach b, $(boards-per-dir), \
+		printf "  %-24s - Build for %s\\n" $*/$(b) $(subst _defconfig,,$(b));) \
+		echo '')
+
+
 # Documentation targets
 # ---------------------------------------------------------------------------
 %docs: scripts_basic FORCE