[cmds] Port ttytetris to ELKS

[ghaerr]

This game is lots smaller than the Nano-X nxtetris, at just 6K bytes. It also doesn't use curses.

Currently, ELKS doesn't implement setitimer so timeout delay is a bit long at 1 second intervals using alarm(1).

To play, use h and l to shift left/right, j and k to spin block. s steps one down and space drops the block.
q quits.

Both ttytetris and ttyclock will run on the serial console too.

Distributed on 360k image, just for @toncho11! :) There's zero bytes free.