Start using Kconfig to configure SeaBIOS settings.

Create autoconf.h during the build.

Move a couple of config settings from config.h to Kconfig.
diff --git a/Makefile b/Makefile
index c7dfc0b..3f89caf 100644
--- a/Makefile
+++ b/Makefile
@@ -115,16 +115,18 @@
 	@echo "  Precompiling $@"
 	$(Q)$(CPP) -P -D__ASSEMBLY__ $< -o $@
 
+$(OUT)asm-offsets.s: $(OUT)autoconf.h
+
 $(OUT)asm-offsets.h: $(OUT)asm-offsets.s
 	@echo "  Generating offset file $@"
 	$(Q)./tools/gen-offsets.sh $< $@
 
 
-$(OUT)ccode.16.s: ; $(call whole-compile, $(CFLAGS16) -S, $(addprefix src/, $(SRC16)),$@)
+$(OUT)ccode.16.s: $(OUT)autoconf.h ; $(call whole-compile, $(CFLAGS16) -S, $(addprefix src/, $(SRC16)),$@)
 
-$(OUT)code32seg.o: ; $(call whole-compile, $(CFLAGS32SEG), $(addprefix src/, $(SRC32SEG)),$@)
+$(OUT)code32seg.o: $(OUT)autoconf.h ; $(call whole-compile, $(CFLAGS32SEG), $(addprefix src/, $(SRC32SEG)),$@)
 
-$(OUT)ccode32flat.o: ; $(call whole-compile, $(CFLAGS32FLAT), $(addprefix src/, $(SRC32FLAT)),$@)
+$(OUT)ccode32flat.o: $(OUT)autoconf.h ; $(call whole-compile, $(CFLAGS32FLAT), $(addprefix src/, $(SRC32FLAT)),$@)
 
 $(OUT)code16.o: romlayout.S $(OUT)ccode.16.s $(OUT)asm-offsets.h
 	@echo "  Compiling (16bit) $@"
@@ -200,6 +202,12 @@
 export KCONFIG_AUTOHEADER := autoconf.h
 export KCONFIG_CONFIG     := $(CURDIR)/.config
 
+$(OUT)autoconf.h : $(KCONFIG_CONFIG)
+	$(Q)$(MAKE) silentoldconfig
+
+$(KCONFIG_CONFIG):
+	$(Q)$(MAKE) defconfig
+
 %onfig:
 	$(Q)mkdir -p $(OUT)/tools/kconfig/lxdialog
 	$(Q)mkdir -p $(OUT)/include/config