scripts: west_commands: runners: Fix default value for --dt-flash

The '--dt-flash' parameter accepts a string like "y", "yes", "no", etc,
and is supposed to be converted into a boolean value. This is only the
theory as in practice, the default value is set to 'n' and is never
converted to False afterwards.

Set the default value to False.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>