kbuild: Fix make help-<board series> on powerpc

make ARCH=powerpc help-<board series> should not require a cofigured
source tree. Also, sort the boards in the output.
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -483,7 +483,7 @@ version_h := include/generated/uapi/linux/version.h
 old_version_h := include/linux/version.h
 
 no-dot-config-targets := clean mrproper distclean \
-			 cscope gtags TAGS tags help %docs check% coccicheck \
+			 cscope gtags TAGS tags help% %docs check% coccicheck \
 			 $(version_h) headers_% archheaders archscripts \
 			 kernelversion %src-pkg
 
@@ -1325,7 +1325,7 @@ help-board-dirs := $(addprefix help-,$(board-dirs))
 
 help-boards: $(help-board-dirs)
 
-boards-per-dir = $(notdir $(wildcard $(srctree)/arch/$(SRCARCH)/configs/$*/*_defconfig))
+boards-per-dir = $(sort $(notdir $(wildcard $(srctree)/arch/$(SRCARCH)/configs/$*/*_defconfig)))
 
 $(help-board-dirs): help-%:
 	@echo  'Architecture specific targets ($(SRCARCH) $*):'