Add "handler" parameter to barebox_update() and remove the code that
was respondible for header lookup before. With this change finding
appropriate handler is caller's responsibility, which makes it
possible to implement custom handler lookup/existence check, chache
it, and then re-use it without calling handler_find_by_* functions for
the second time.

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