tools: lib: thermal: expose thermal_exit symbols

Remove duplicate entry for thermal_init and add the missing entries for
thermal_exit and their respectives in cmd, events, and sampling context.

Signed-off-by: Emil Dahl Juhl <juhl.emildahl@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>