Makefiles: remove @true

Apparently @true was used as a rule for any target which should have
a non-empty rule doing nothing. For this, there's a "<target>: ;"
syntax, let's use this and eliminate an unnecessary fork/exec :)

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>