make: have `git submodule update` be recursive

libwally added libsecp256k1 as a submodule, so we need to recursively
update the submodules now

doc/INSTALL.md

@@ -385,7 +385,7 @@ Clone lightning:
 ```
 git clone https://github.com/ElementsProject/lightning.git
 cd lightning
-git submodule update --init
+git submodule update --init --recursive
 ```
 Build and install:
 ```

tools/refresh-submodules.sh

@@ -38,5 +38,5 @@ if [ "$(git submodule status "$@" | grep -c '^ ')" != $# ]; then
     echo Reinitializing submodules "$@" ...
     git submodule sync "$@"
     rm -rf "$@"
-    git submodule update --init "$@"
+    git submodule update --init --recursive "$@"
 fi