Add list of enabled modules to welcome message

[dimitrit]

Fixes #31

[marcelstoer]

Looks just fine, thanks. Can you provide a sample boot message from the firmware you tested with?

[dimitrit]

Here are a couple of samples showing the module names in the boot message:

• Random selection of user modules, including ones with a comment (ENDUSER_SETUP) and leading white-space (TCS34725):

  NodeMCU 2.2.0.0 built with Docker provided by frightanic.com
  branch: master
  commit: 67027c0d05f7e8d1b97104e05a3715f6ebc8d07f
  SSL: false
  modules: bit,enduser_setup,file,gpio,net,node,tcs34725,uart,wifi
  build created on 2018-04-07 18:07
  powered by Lua 5.1.4 on SDK 2.2.1(cfd48f3)
  lua: cannot open init.lua
  > 

• A dash is shown when no user modules are enabled:

  NodeMCU 2.2.0.0 built with Docker provided by frightanic.com
  branch: master
  commit: 67027c0d05f7e8d1b97104e05a3715f6ebc8d07f
  SSL: false
  modules: -
  build created on 2018-04-07 18:14
  powered by Lua 5.1.4 on SDK 2.2.1(cfd48f3)
  lua: cannot open init.lua
  > 

[marcelstoer]

Thanks a lot Dimitri for your dedication and the thorough testing!