lxc: set RUNTIME_PATH define to the /var/run path

lxc: set RUNTIME_PATH define to the /var/run path

The default runtime directory used by LXC is /run which doesn't exist
in OpenWrt. It causes errors like:

Failed to create lock for foo
lxc-create: foo: tools/lxc_create.c: main: 260 Failed to create lxc container

There has been workaround for that in the lxc-auto.init but it requires
installing "lxc-auto" package. Replacing that "ln -s" workaround with
Makefile specifying RUNTIME_PATH define allows using pure "lxc" in
OpenWrt (without the "lxc-auto").

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 90fef036fe465262d5915489d45f430b313f22ab)