Documentation/Makefile: set nmkdir

I was not able to find a way to make a target in sub-makefile
(such as "cd Documentation && make pdf") without this hack.

If there's a better way, please let me know

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Documentation/Makefile

+__nmk_dir ?= ../scripts/nmk/scripts/
 include $(__nmk_dir)/include.mk
 include $(__nmk_dir)/macro.mk