When BUILD_NBUILDS is > 1 we run the tidy command.  With the addition of
DocBook this now includes a -C doc/DocBook and a 'entering/leaving' pair
of messages happen.  Since we don't want to see what's being cleaned
here, we can just invoke make -s like we do when building.

Signed-off-by: Tom Rini <trini@ti.com>