Fix break in luac.cross build and add some documentation

app/include/module.h

@@ -67,7 +67,7 @@
   const LOCK_IN_SECTION(".lua_rotable") \
     luaR_table MODULE_PASTE_(lua_rotable_,name) = { luaname, map }
 
-#if !(MIN_OPT_LEVEL==2 && LUA_OPTIMIZE_MEMORY==2)
+#if !defined(LUA_CROSS_COMPILER) && !(MIN_OPT_LEVEL==2 && LUA_OPTIMIZE_MEMORY==2)
 # error "NodeMCU modules must be built with LTR enabled (MIN_OPT_LEVEL=2 and LUA_OPTIMIZE_MEMORY=2)"
 #endif
 

app/modules/linit.c

@@ -57,7 +57,12 @@ extern const luaR_entry math_map[];
 BUILTIN_LIB(      MATH,      LUA_MATHLIBNAME,   math_map);
 #endif
 
+#ifdef LUA_CROSS_COMPILER
+const luaL_Reg lua_libs[] = {{NULL, NULL}};
+const luaR_table lua_rotable[] = {{NULL, NULL}};
+#else 
 extern const luaL_Reg lua_libs[];
+#endif
 
 void luaL_openlibs (lua_State *L) {
   const luaL_Reg *lib = lua_libs;

docs/en/upload.md

@@ -35,4 +35,23 @@ Supported platforms: Windows
 
 Source: [https://github.com/4refr0nt/luatool](https://github.com/4refr0nt/luatool)
 
-Supported platforms: OS X, Linux, Windows, anything that runs Python
+Supported platforms: OS X, Linux, Windows, anything that runs Python
+
+# Compiling Lua on your PC for Uploading
+
+If you install lua on your development PC or Laptop then you can use the standard Lua
+compiler to syntax check any Lua source before downloading it to the ESP8266 module.  However,
+the nodemcu compiler output uses different data types (e.g. it supports ROMtables) so the
+compiled output cannot run on the ESP8266.  
+
+Compiling source on one platform for use on another (e.g. Intel x38 Window to ESP8266) is 
+known as _cross-compilation_ and the nodemcu firmware supports the compilation of `luac.cross` 
+on \*nix patforms which have Lua 5.1, the Lua filesystem module (lfs), and the essential
+GCC tools.  Simply change directory to the firmware root directoy and run the command:
+
+    lua tools/cross-lua.lua
+    
+This will generate a `luac.cross` executable in your root directory which can be used to
+compile and to syntax-check Lua source on the Development machine for execution under 
+nodemcu lua on the ESP8266. 
+