Expose bbu_find_handler_by_device() and bbu_find_handler_by_name() as
public functions and convert the only user of
barebox_update_handler_exists() to use the former function instead.

With this done, barebox_update_handler_exists() is no longer used
anywhere in the code and can be removed.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>