I'm playing with barebox on an Apple M1-based MacBook Pro that uses
the (new-ish) x2r10g10b10 framebuffer layout. The attached patch is
sufficient to get a white-on-black prompt rather than black-on-black
text.

(I can't actually use the prompt yet, as there's no keyboard driver,
but it's a start.)

I'm aware that this code isn't as accurate as it could be (the LSBs of
the 10-bit color are always 00), but I think it's worth it to keep
this code (relatively) simple, and the difference is unlikely to ever
be apparent to anyone. Please let me know if you'd prefer a version
which does the (r * 0x101) >> (16 - length) thing or any other
version.

Thanks!
Pip