Enable the preservation of configuration items

This commit allows configuring the items in advance by executing
"make config" and specifying them in a configuration file. Subsequently,
run "make" according to the provided configurations. e.g.,
  $ make config ENABLE_SDL=0
  $ make

Author: jserv

Makefile

@@ -4,6 +4,8 @@ include mk/toolchain.mk
 OUT ?= build
 BIN := $(OUT)/rv32emu
 
+-include $(OUT)/.config
+
 CFLAGS = -std=gnu99 -O2 -Wall -Wextra
 CFLAGS += -Wno-unused-label
 CFLAGS += -include src/common.h
@@ -120,6 +122,10 @@ $(BIN): $(OBJS)
 	$(VECHO) "  LD\t$@\n"
 	$(Q)$(CC) -o $@ $^ $(LDFLAGS)
 
+config:
+	$(Q)echo "$(CFLAGS)" | xargs -n1 | sort | sed -n 's/^RV32_FEATURE/ENABLE/p' > $(OUT)/.config
+	$(VECHO) "Check the file $(OUT)/.config for configured items.\n"
+
 # Tools
 include mk/tools.mk
 tool: $(TOOLS_BIN)
@@ -176,5 +182,6 @@ distclean: clean
 	$(RM) -r $(OUT)/id1
 	$(RM) *.zip
 	$(RM) -r $(OUT)/mini-gdbstub
+	-$(RM) $(OUT)/.config
 
 -include $(deps)

README.md

@@ -80,6 +80,14 @@ The usage and limitations of Doom and Quake demo are listed in [docs/demo.md](do
 
 e.g., run `make ENABLE_EXT_F=0` for the build without floating-point support.
 
+Alternatively, configure the above items in advance by executing `make config` and
+specifying them in a configuration file. Subsequently, run `make` according to the provided
+configurations. For example, employ the following commands:
+```shell
+$ make config ENABLE_SDL=0
+$ make
+```
+
 ### RISCOF
 
 [RISCOF](https://github.com/riscv-software-src/riscof) (RISC-V Compatibility Framework) is