boot: cover for hardware keys on phones/tablets

boot: cover for hardware keys on phones/tablets

The patch is originally from Brenton Simpson, I (Lennart) just added some
comments and rebased it.

I didn't test this, but the patch looks so obviously right to me, that
I think we should just merge it, instead of delaying this further. In
the worst case noone notices, in the best case this makes sd-boot work
reasonably nicely on devices that only have a hadware power key + volume
rocker.

Fixes: #30598
Replaces: #31135