apps,litenes,Makefile: add rules for generating ROM source files

* The shell commands are evaluated before rule execution. If we run
  `make` for the first time, the `$SRCS` will not include the ROM source
  files, which leads to linking error.
* Now we add a rule to first manually generate the ROM source files.

apps/litenes/Makefile

@@ -1,8 +1,9 @@
 NAME := litenes
 SRCS := $(shell find -L ./src/ -name "*.c")
 ROMS := $(shell find -L ./src/ -name "*.nes")
-PREBUILD := src/roms/gen/roms.h
+PREBUILD := rom
 include $(AM_HOME)/Makefile.app
 
-src/roms/gen/roms.h: $(ROMS)
-	@cd src/roms && python3 build-roms.py
+.PHONY: rom
+rom: $(ROMS)
+	@cd src/roms && python3 build-roms.py