Use sterm instead of cu on ryuglab target

Compared to cu, sterm does not wait 2 seconds after hangup before it exits
(and leaves the port for another user/connection) and can reset the board
via command line switches (no need for --reset-cmd).

Sterm lives at
https://rtime.felk.cvut.cz/gitweb/can-benchmark.git/blob/HEAD:/utils/sterm.c

Additionally, use ssh -tt instead of -t. This causes the sterm to receive
SIGHUP even if ssh does not run on terminal.