diff --git a/scripts/prepare_release.sh b/scripts/prepare_release.sh
index 6e6a1b7..9013cc8 100755
--- a/scripts/prepare_release.sh
+++ b/scripts/prepare_release.sh
@@ -1,6 +1,15 @@
 #!/bin/bash
 
-# Prepare .gitignore for a release.
+print_usage()
+{
+    cat <<EOF
+Usage: $0 [OPTION]...
+Prepare the source tree for a release.
+
+Options:
+  -u    Prepare for development (undo the release preparation)
+EOF
+}
 
 # Copyright The Mbed TLS Contributors
 # SPDX-License-Identifier: Apache-2.0
@@ -19,42 +28,29 @@
 
 set -eu
 
-print_usage()
-{
-    echo "Usage: $0"
-    echo -e "  -h|--help\t\tPrint this help."
-    echo -e "  -i|--ignore\t\tAdd generated files to the gitignores."
-    echo -e "  -u|--unignore\t\tRemove generated files from the gitignores."
-}
-
-if [[ $# -eq 0 ]]; then
+if [ $# -ne 0 ] && [ "$1" = "--help" ]; then
     print_usage
-    exit 1
-elif [[ $# -ge 2 ]]; then
-    echo "Too many arguments!"
-    exit 1
+    exit
 fi
 
-case "$1" in
-    -i | --ignore)
-        IGNORE=true
-        ;;
-    -u | --uignore)
-        IGNORE=false
-        ;;
-    -h | --help | "")
-        print_usage
-        exit 1
-        ;;
-    *)
-        echo "Unknown argument: $1"
-        echo "run '$0 --help' for options"
-        exit 1
-esac
+unrelease= # if non-empty, we're in undo-release mode
+while getopts u OPTLET; do
+    case $OPTLET in
+        u) unrelease=1;;
+        \?)
+            echo 1>&2 "$0: unknown option: -$OPTLET"
+            echo 1>&2 "Try '$0 --help' for more information."
+            exit 3;;
+    esac
+done
+
+
+
+#### .gitignore processing ####
 
 GITIGNORES=$(find . -name ".gitignore")
 for GITIGNORE in $GITIGNORES; do
-    if $IGNORE; then
+    if [ -n "$unrelease" ]; then
         sed -i '/###START_COMMENTED_GENERATED_FILES###/,/###END_COMMENTED_GENERATED_FILES###/s/^# //' $GITIGNORE
         sed -i 's/###START_COMMENTED_GENERATED_FILES###/###START_GENERATED_FILES###/' $GITIGNORE
         sed -i 's/###END_COMMENTED_GENERATED_FILES###/###END_GENERATED_FILES###/' $GITIGNORE
@@ -64,3 +60,23 @@
         sed -i 's/###END_GENERATED_FILES###/###END_COMMENTED_GENERATED_FILES###/' $GITIGNORE
     fi
 done
+
+
+
+#### Build scripts ####
+
+# GEN_FILES defaults on (non-empty) in development, off (empty) in releases
+if [ -n "$unrelease" ]; then
+    r=' yes'
+else
+    r=''
+fi
+sed -i 's/^\(GEN_FILES[ ?:]*=\)\([^#]*\)/\1'"$r/" Makefile */Makefile
+
+# GEN_FILES defaults on in development, off in releases
+if [ -n "$unrelease" ]; then
+    r='ON'
+else
+    r='OFF'
+fi
+sed -i 's/^\( *option *( *GEN_FILES  *"[^"]*"  *\)\([A-Za-z0-9][A-Za-z0-9]*\)/\1'"$r/" CMakeLists.txt