[U-Boot] Porting UBI fixes (specially fastmap's) to U-Boot

Hello Heiko,

According to Richard Weinberger, UBI fastmap is broken in U-Boot. There are plenty of fixes in Linux that we should pull in U-Boot to fix it.

Maybe you are already aware of this and you have some work-in-progress patches to share? Otherwise, I might be able to spend some time working on this in the next weeks.

Thanks!