N900: upgrade kernel

The Nokia N900 kernel is close to mainline, but hasn't seen an update in quite a while. It's in the community category of postmarketOS devices, so let's upgrade the kernel to something more recent and fix these two things:

• Annoying mmc1: unspecified timeout for CMD6 -- use generic scrolling through while booting
  • actually... do we need to display kernel output at all? I guess it looks much better if we hide it altogether during boot on this device, like we do on the pinephone and others.
• the SD card only shows up if the backcover is closed. Somebody from Maemo Leste submitted a patch upstream that disables this behavior. This is something we run into from time to time when testing, it looks like the booting process is broken.

Note that there are a lot of n900-related issues open, some as old as 3 years. It would be great to go through them and close the ones that are no longer relevant, which is probably most of them.

Tagging maintainers of the N900:

• @MartijnBraam
• @drebrez

Or maybe @craftyguy wants to work on it for the next service pack?