[U-Boot] Silence "make clean"

[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.

Best regards,

Wolfgang Denk

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