[U-Boot] Kconfig menu layout

[Alessandro Rubini]

> There is many vendors which have just a single board;

Like the ST nomadik8815 I maintain.

> Just adding an additional directory level for a single entry does
> not make much sense to me.

So should we leave the nomadik as a single-entry vendor board
or leave it as is? (more nomadik boards will be added over time, but none of
them is in the queue nor will be in the foreseeable months)

/alessandro