Rename fatvbr -> fat, generalize makefile

Makefile

@@ -31,16 +31,13 @@ kernel.com: kernel/*.asm lib/*.asm
 debug.rom: debug/*.asm lib/*.asm
 	nasm -s -o $@ -I lib -I debug debug/main.asm && scripts/fix-rom.sh $@
 
-boot/floppy.bs: boot/fatvbr.asm
-	nasm -s -DCMDLINE='"KERNEL.COM"' -o $@ $<
-
-boot/mbr.bs: boot/mbr.asm
-	nasm -s -DFLOPPY=$(FLOPPY) -o $@ $<
+boot/%.bs: boot/%.asm
+	nasm -s -DFLOPPY=$(FLOPPY) -DCMDLINE='"KERNEL.COM"' -o $@ $<
 
 programs/%.com: programs/%.asm
 	nasm -s -I lib -o $@ $<
 
-fdimage.img: boot/floppy.bs $(DISTFILES)
+fdimage.img: boot/fat.bs $(DISTFILES)
 	mformat -C -i $@ -f $(FLOPPY) -B boot/floppy.bs ::
 	mcopy -i $@ $(DISTFILES) ::