Hi Christian,
first of all thanks for your patch.
I think it is great to have a better/smaller font. However, I cannot merge the patch as-is, as I have the following comments:
1) we need a way to select between individual fonts when writing to the display. I think in the long run it is inevitable that we'll use small and big letters on screen and have to select/ switch between them. Something like display_select_font(FONT_5x8) to be called before a display_puts() sounds like a reasonable API for this.
2) Removing all the special characters might not be the best idea to do. If at all, it should be a compile time option whether or not to drop the special characters. Also, the check for replacing a character with '?' needs to be a font-specific and not a global decision.
3) introducing the new font should be one patch, fixing the 'miss topmost line' another patch, and any experimental changes to hello_world/main.c should be yet another patch. This way we can keep the commitlog a bit more clean.
I hope you're willing to rework your code, thanks in advance!
Harald