boards/pinetime: add defines for controlling the backlight pin

[aabadie]

Contribution description

This PR adds some defines (BACKLIGHT_ON, BACKLIGHT_OFF) for controlling the Pinetime backlight pin.
These defines are meant to be generic, similar to on-board LED macros, and should be defined in boards that need it (the adafruit-clue needs it for example).

Testing procedure

• Check the tests/driver_ili9341, tests/disp_dev and tests/pkg_lvgl are displaying something on the pinetime
• Check that with any other applications (examples/hello-world) the screen remains off.

Issues/PRs references

Will help with #13276

[kaspar030]

Why defines? (Why are defines creeping into the codebase lately?)
What's wrong with a proper function?

[aabadie]

What's wrong with a proper function?

I'm adding one in disp_dev. But we need a common define at board level.

[bergzand]

Why defines? (Why are defines creeping into the codebase lately?)

We need a way to 1. define a common pin and 2. define whether it is active low or active high. The current macros allow for that, but a set of common defines to select a gpio_t and indicate active high or low would also work. A new param struct for this seems a bit overkill I think.

[bergzand]

@kaspar030 Are you okay with the current changes here or do you have a different proposal?

[bergzand]

@kaspar030 Are you okay with the current changes here or do you have a different proposal?

@kaspar030 mentioned offline that he is fine with the current changes as it is an iprovement of the current state, but that we're going to run into issues with multiple displays and when hooking up a display to an existing board.

[bergzand]

@aabadie Please squash!

[bergzand]

Ack!

[aabadie]

Squashed!