Update config files: modularize BLK_DEV_MD, disable MD_AUTODETECT
    
    Michal Marek says:
    I think that the reason why BLK_DEV_MD is built in is
    
    config MD_AUTODETECT
            bool "Autodetect RAID arrays during kernel boot"
            depends on BLK_DEV_MD=y
    
    but since we compile the raid presonalities as modules, this cannot
    work. Also, we use the initrd even in the trivial cases, so why bother
    about root-on-md.