Switch activity LED blinking from delay to polling

[morio]

The purpose behind this change is to stop having USB console logging be jittery because of blinking function when there is no SD card inserted. This was due to the blinking function being delay based. Switched the blink function to polling and checks when blinking should occur with deadlines.

[PetteriAimonen]

Seems like a reasonable approach. I think the "led override" functions could use a few comments on what the purpose is and how it works.

This will also cause more frequent reinitialization attempts of the SD card, but I think that shouldn't cause any harm.

A lower effort/impact way to the same goal would be to make the blinking function do something like:

for (int i = 0; i < time/10; i++)
{
   delay(10);
   platform_poll();
}

instead of the plain delay.

[aperezbios]

Thanks for the feedback, @PetteriAimonen. Merging with updates.