platforms/run_model.sh: update headless execution mode

Headless execution is a mode where the visualisation windows are
disabled, thus no automatic pop-up of xterm with logs; the model is
still executing in the background and booting as normal.

This change renames the variable 'automate' to 'headless' and the flag
'-j' that controls this selection no longer requires passing true or
false after being set. Simply, if the flag is used, headless mode is
enabled.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: Id4449ac464a9a8075b907c8f8d0d307f242c79e8