feat: Introducing headless and termination modes

This change modifies the behaviour of headless mode '-j' flag, to not
require the value true or false so that if the flag is set the model
executes in headless mode.

Also introduces '-t' flag to control model termination if set; this is
required as some test suites require headless mode execution but need
the model to keep executing after a boot complete is detected.

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