Remove minnsh configurations and support logic: up_getc() and lowinstream.

This was an interesting exercise to see just how small you could get NuttX, but otherwise it was not useful:  (1) the NSH code violated the OS interface layer by callup up_getc and up_putc directly, and (2) while waiting for character input, NSH would call up_getc() which would hog all of the CPU.  NOt a reasonably solution other than as a proof of concept.