config: add support for <custom>_defconfig

Add support for platform specific and/or custom defconfig
per arch. They have to be suffixed by '_defconfig' and are
automatically detected and displayed by issuing 'make ARCH=<arch> help'.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

Makefile.help

@@ -28,6 +28,12 @@ help:
 	@echo '  oldconfig		- resolve any unresolved symbols in .config'
 	@echo '  silentoldconfig	- resolve any unresolved symbols in .config, silently'
 	@echo '  randconfig		- generate a random .config'
+	@$(if $(arch-defconfigs), \
+		@echo ''; \
+		echo 'Architecture specific configs ($(ARCH))'; \
+		$(foreach c, $(arch-defconfigs), \
+			printf "  %-21s - Build for %s\\n" $(c) $(subst _defconfig,,$(c));) \
+	)
 	@echo
 	@echo 'Installation:'
 	@echo '  install		- install both the runtime and the headers'

Makefile.in

@@ -422,6 +422,8 @@ $(addprefix $(top_builddir),include include/bits include/sys include/config lib
 $(conf) $(mconf): | $(top_builddir)include/config $(top_builddir)extra/config/lxdialog
 	$(Q)$(MAKE) -C extra/config $(@F)
 
+arch-defconfigs := $(notdir $(wildcard $(top_srcdir)extra/Configs/defconfigs/$(ARCH)/*_defconfig))
+
 menuconfig: $(mconf)
 	$(Q)$< extra/Configs/Config.in
 
@@ -453,6 +455,9 @@ allnoconfig: $(conf)
 defconfig: $(conf)
 	$(Q)$< -D extra/Configs/defconfigs/$(ARCH)/$@ extra/Configs/Config.in
 
+%_defconfig: $(conf)
+	$(Q)$< -D extra/Configs/defconfigs/$(ARCH)/$@ extra/Configs/Config.in
+
 menuconfig-clean-y:
 	$(Q)$(MAKE) -C extra/config CLEAN_extra/config