make: Don't set $(MAKEFLAGS)

We shouldn't set MAKEFLAGS by the following reasons:
1. User may want to specify some make parameter (e.g., `-d` for debug)
2. We lose parallel build. No `-j` is passed to submake and it looks
   like, gnu/make will not deal with parallel recursive make if
   $(MAKEFLAGS) is unset back.
   Easy to verify: Add `sleep 3` to build rule in Makefile.inc and
   you'll find only one sleep process at a time. After the patch
   if you specify say `-j5` to make - you'll have 5 sleep processes.

Reverts: commit e9beed7b ("build: zdtm -- Add implicit rules into
zdtm building").

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Makefile

@@ -278,11 +278,11 @@ docs:
 .PHONY: docs
 
 zdtm: all
-	$(Q) MAKEFLAGS= $(MAKE) -C test/zdtm all
+	$(Q) $(MAKE) -C test/zdtm all
 .PHONY: zdtm
 
 test: zdtm
-	$(Q) MAKEFLAGS= $(MAKE) -C test
+	$(Q) $(MAKE) -C test
 .PHONY: test
 
 #