poller: Fix async poller ...

The async poller does not work as expected since it can happen that
the async poller is removed from the list of pollers while we are
iterating over the list. Even list_for_each_entry_safe does not help
here since we may remove the next list element, but
list_for_each_entry_safe only allows to remove the current list element.

Rework the async poller so that it is registered with the poller
framework on registration and then is only marked as active with
poller_call_async(). This way we do not have to do list manipulations
while running the pollers.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

poller: Allow to call functions asynchronously ...

Sometimes execution of a function has to be delayed, for example
when a backlight can only be turned on when the picture has stabilized.
To help in such situations add a convenience function around the poller
stuff to call a function after a delay.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add generic poll infrastructure ...

Barebox does not have interrupt functionality. Nevertheless it's
sometimes useful to periodically call functions, like for example
a heartbeat LED or watchdog reset. Instead of cluttering the code
with calls to these functions this patch adds a generic polling
infrastructure. Code which might run for longer now can call
poller_call() periodically which in turn will call all registered
pollers.
This patch adds a call to poller_call in two generic pathes. First
of them is getc() which covers waiting for uart input. Second is
ctrlc() which should be called anyway from code which might run
for longer. So instead adding poller_call directly to your code,
consider checking ctrlc instead which also gives additional
convenience to the user.
The poller code is safe against reentrancy which means that it's
safe to call poller_call inside a poller.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>