Add support to use NuttX as POSIX PSE51 system

NuttX is a very complete POSIX RTOS, but sometimes for low end applications like some low cost automotive application POSIX standard is overwhelming.

This issue was fixed with PSE51 subset, that doesn't require many POSIX features, like file systems, etc.

Some years ago I removed many of NuttX features to make it running in a low end DIP package LPC1114 MCU, picture here:
https://flickr.com/photos/79703543@N00/18013069041/in/dateposted-public/

After some time this minnsh demo was removed from NuttX mainline and many options to disable feature also were removed from kernel (option to disable FILES, VFS, etc). It is important to reintroduce these options to disable NuttX features to make it adaptable to POSIX PSE51 standard.