drivers/periph_common: add periph_init_buttons to init on-board buttons

[benpicco]

Contribution description

This adds a periph_init_buttons module analogous to the periph_init_leds.
However, unlike the LED module, this is not a DEFAULT_MODULE as most applications that make use of the button will take care of initializing it themselves (e.g. as an interrupt).

There is also no consistency with only a few boards auto-initializing the button.
Now with the new module, applications that rely on button auto-init can request it reliably.

Testing procedure

There is no in-tree code that makes use of this feature.
But in it's simplest form, this can be tested with

#include "board.h"
#include "periph/gpio.h"

int main(void)
{
    while (1) {
        gpio_write(LED0_PIN, gpio_read(BTN0_PIN));
    }

    return 0;
}

with USEMODULE += periph_init_buttons

Issues/PRs references

similar to #17584

[fjmolinas]

LGTM, how about adding it by default to those BOARDS that were initializing buttons? That way there is no change for any user relying on that? (it also adds a compile test for the module)

[benpicco]

how about adding it by default to those BOARDS that were initializing buttons?

I don't know, most of those will be just copy & paste without any deeper meaning.
The goal of this PR was to get rid of that very chaos and establish a unified behavior across all boards.

If this really breaks something, I hope people will notice 😉

[fjmolinas]

Why add this test? there is already a tests/leds and tests/buttons.

[fjmolinas]

I don't this test is needed, better adapt the already present tests no?

[benpicco]

Ok, I added it to tests/leds

[fjmolinas]

Can you extended the README accordingly?|

[benpicco]

Sure!

[fjmolinas]

ACK