Fix for update script when submodule is not initialized

Signed-off-by: Jan-Christoph Borchardt <hey@jancborchardt.net>

build/update.sh

@@ -11,7 +11,7 @@ git --no-pager log -3 --pretty=format:"%h %Cblue%ar%x09%an %Creset%s"
 printf "\n"
 git branch --merged master | grep -v "master$" | xargs --no-run-if-empty git branch -d
 git fetch --prune --quiet
-git submodule update
+git submodule update --init
 
 # Update apps
 source ./build/update-apps.sh