Makefile: Add a target to create the .cfg files

A common requirement when converting CONFIG options to Kconfig is to check
that the effective configuration has not changed due to the conversion. Add
a target which creates this configuration (in the form of u-boot.cfg) but
does not build U-Boot. This speeds up the checking.

Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -821,6 +821,8 @@ append = cat $(filter-out $< $(PHONY), $^) >> $@
 quiet_cmd_pad_cat = CAT     $@
 cmd_pad_cat = $(cmd_objcopy) && $(append) || rm -f $@
 
+cfg: u-boot.cfg
+
 all:		$(ALL-y)
 ifeq ($(CONFIG_DM_I2C_COMPAT)$(CONFIG_SANDBOX),y)
 	@echo "===================== WARNING ======================"
@@ -1527,6 +1529,7 @@ help:
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  ubootrelease	  - Output the release version string (use with make -s)'
 	@echo  '  ubootversion	  - Output the version stored in Makefile (use with make -s)'
+	@echo  "  cfg		  - Don't build, just create the .cfg files"
 	@echo  ''
 	@echo  'Static analysers'
 	@echo  '  checkstack      - Generate a list of stack hogs'