Generate rconfig.sys from distfile

*.sys files are affected by make clean.

Makefile

@@ -1,7 +1,7 @@
 FLOPPY = 360
 
 PROGRAMS = $(patsubst %.asm,%.com,$(wildcard programs/*.asm))
-DISTFILES = rdos.sys kernel/rconfig.sys $(PROGRAMS)
+DISTFILES = rdos.sys rconfig.sys $(PROGRAMS)
 ROMS =
 
 QEMU_ARCH = $(shell uname -m)
@@ -28,6 +28,9 @@ default: rdos.sys
 rdos.sys: kernel/*.asm lib/*.asm
 	nasm -s -o $@ -l kernel.lst -I lib -I kernel kernel/main.asm
 
+rconfig.sys: config.txt
+	cp $< $@
+
 debug.rom: debug/*.asm lib/*.asm
 	nasm -s -o $@ -I lib -I debug debug/main.asm && scripts/fix-rom.sh $@