Add DirectFB demo examples on NuttX

[acassis]

Recently (OK! one month ago) @caramelli add support for NuttX on DirectFB: https://github.com/directfb2/DirectFB2/pull/107/commits/d11102daba0af1f75339d9a27b3b5c9eba242884

It is LGPL, but there is not linking exception as FreeRTOS used to have when it was LGPL.

BTW even with this restriction it could be useful for people using to demonstrate Linux applications running on NuttX.

[xiaoxiang781216]

Should we mainline @caramelli 's work?:)

[acassis]

Yes, I think it should be nice to encourage more people to test it! :-D