Create a .cfg file containing the CONFIG options used to build

At present CONFIG options are split across Kconfig and board config headers
files. Also we have multiple files containing these CONFIG options.

In order to see exactly what is being used for building, create a .cfg
file which holds these options as reported by the C preprocessor.

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

scripts/Makefile.spl

@@ -149,7 +149,7 @@ endif
 boot.bin: $(obj)/u-boot-spl.bin
 	$(call if_changed,mkimage)
 
-ALL-y	+= $(obj)/$(SPL_BIN).bin
+ALL-y	+= $(obj)/$(SPL_BIN).bin $(obj)/$(SPL_BIN).cfg
 
 ifdef CONFIG_SAMSUNG
 ALL-y	+= $(obj)/$(BOARD)-spl.bin
@@ -165,6 +165,13 @@ endif
 
 all:	$(ALL-y)
 
+quiet_cmd_cpp_cfg = CFG     $@
+cmd_cpp_cfg = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi \
+		-D__ASSEMBLY__ -x assembler-with-cpp -P -dM -E -o $@ $<
+
+$(obj)/$(SPL_BIN).cfg:	include/config.h
+	$(call if_changed,cpp_cfg)
+
 ifdef CONFIG_SAMSUNG
 ifdef CONFIG_VAR_SIZE_SPL
 VAR_SIZE_PARAM = --vs