openthread-br: fix build error on some platforms

On some builders the package was failing to build:
```
cp: cannot overwrite non-directory '[..]/var' with directory '[..]/var
```

Here we're moving the creation of /var/lib/thread into runtime script,
which eliminates the error.

Signed-off-by: krant <aleksey.vasilenko@gmail.com>

net/openthread-br/Makefile

@@ -96,8 +96,7 @@ define Package/openthread-br/install
 	$(INSTALL_DIR) \
 		$(1)/etc/init.d \
 		$(1)/lib/netifd/proto \
-		$(1)/usr/sbin \
-		$(1)/var/lib/thread
+		$(1)/usr/sbin
 	$(INSTALL_BIN) ./files/openthread-proto.sh $(1)/lib/netifd/proto/openthread.sh
 	$(INSTALL_BIN) $(PKG_INSTALL_DIR)/usr/sbin/* $(1)/usr/sbin
 endef

net/openthread-br/files/openthread-proto.sh

@@ -88,6 +88,7 @@ proto_openthread_setup() {
 	}
 
 	json_for_each_item proto_openthread_add_prefix prefix
+	mkdir -p /var/lib/thread
 	ubus call otbr threadstart || proto_openthread_setup_error "$interface" MISSING_UBUS_OBJ
 	$OTCTL netdata register