Using the NODEMCU_ namespace prefix makes it obvious that these are not
part of Lua proper (contrast, e.g., LUA_BUILTIN_STRING). Using
"CMODULE" gives us room to differentiate between modules whose
implementation is in C and whose implemenation is in Lua ("LMODULE").
The ESP8266 branch can adopt the same convention when it moves to
Kconfig; see https://github.com/nodemcu/nodemcu-firmware/issues/3130
dd3b217df2