Support configuring Busybox from _defconfig files

The existing test .config files are moved to configs/ and renamed
to *_defconfig. 'make xyz_defconfig' will enable the
configuration in that specific file.

Signed-off-by: Daniel Fandrich <dan@coneharvesters.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Makefile

@@ -1042,7 +1042,7 @@ rpm: FORCE
 # Brief documentation of the typical targets used
 # ---------------------------------------------------------------------------
 
-boards := $(wildcard $(srctree)/arch/$(ARCH)/configs/*_defconfig)
+boards := $(wildcard $(srctree)/configs/*_defconfig)
 boards := $(notdir $(boards))
 
 -include $(srctree)/Makefile.help
@@ -1131,15 +1131,6 @@ clean: $(clean-dirs)
 		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \) \
 		-type f -print | xargs rm -f
 
-help:
-	@echo  '  Building external modules.'
-	@echo  '  Syntax: make -C path/to/kernel/src M=$$PWD target'
-	@echo  ''
-	@echo  '  modules         - default target, build the module(s)'
-	@echo  '  modules_install - install the module'
-	@echo  '  clean           - remove generated files in module directory only'
-	@echo  ''
-
 # Dummies...
 PHONY += prepare scripts
 prepare: ;