ARM: remove annoying #warning

Although done with the best intentions, the usage of #warning in the
ARM build makes it hard to see what's going on, or see any "real"
warnings.

Remove at least this particularly often triggered one, so we can
have a relatively "quiet" build again...
ARM: keep all pages we've mapped during kernel startup

Don't just keep the page directory, but also the actual allocates
pages for the pagetables we've created.