include: Add shorthand for simple directory building

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
build: Add @all target

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
test: Use gen-build-dir

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>