x86-generic: add MMC support to boot off SD cards

MMC support has been added to x86-64 a while ago, there is no reason not
to support it in x86-generic as well.

Signed-off-by: Andreas Ziegler <github@andreas-ziegler.de>
[Matthias Schiffer: add commit message]
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>