Updating submodules in the polycube repository

If this script is launched on an existing (and perhaps old) source code repository, submodules may be out of date.
This commits adds a command that updates all the polycube submodules.

Signed-off-by: Fulvio Risso <fulvio.risso@polito.it>

Author: frisso

scripts/install.sh

@@ -142,6 +142,10 @@ if [ "$INSTALL_CLEAN_POLYCUBE" == true ] ; then
   $SUDO rm -rf build
 fi
 
+# Update submodules; useful in case you are re-running
+#   this script on an existing install
+git submodule update --init --recursive
+
 mkdir -p build && cd build
 git log -1