Opentx: Horus: add SD card benchmark on the debug page

[raphaelcoeffic]

That might help to understand some strange issues with slow boot, or slow "model select" page. Maybe adding some warnings if the speed is really too slow (would need some data from users having working setups).

[raphaelcoeffic]

It seems that already exists on X7/X9: press long while in the SD explorer:

[kilrah]

Umm, what provides this value is...

uint32_t sdGetSpeed()
{
  return 330000;
}

[stale[bot]]

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