[U-Boot] Silence "make clean"

[Marek Vasut]

Dear Wolfgang Denk,

> Dear Marek,
> 
> your commit 30ff8918 ("kerneldoc: Implant DocBook from Linux kernel")
> causes "make clean" (and all other targets that use this implicitly)
> to spit out these messages:
> 
> 	make[1]: Entering directory `.../doc/DocBook'
> 	make[1]: Leaving directory `.../doc/DocBook'
> 
> We didn't have this before, and we shouldn't have this.
> 
> Can you please fix this regression?  Thanks.

Is this not fixed in latest sources? I believe Tom fixed it.

> Best regards,
> 
> Wolfgang Denk

Best regards,
Marek Vasut