Add option to rel2fw.sh to allow post-processing

This would typically be invoked by `mix firmware` further up the
stack. It is at the right moment to allow operating on the squashfs
file system before it is packed up into the fw file.

The primary purpose is to allow signing the root filesystem for use
with dm-verity.

scripts/rel2fw.sh

@@ -28,6 +28,7 @@ usage() {
     echo "  -f <firmware output file>  Default is $PROJECT_DIR.fw"
     echo "  -o <image output file>     Default is $PROJECT_DIR.img"
     echo "  -p <file priorities>       The ordering of files in the rootfs (may be specified more than once)"
+    echo "  -s <script path>           Post-processing script for rootfs (optional)"
     echo
     echo "Barring errors, the firmware file is always created. The raw image is only"
     echo "created if specified."
@@ -48,7 +49,7 @@ sbin/init 32764
 etc/erlinit.config 32763
 END
 
-while getopts "a:c:f:o:p:" opt; do
+while getopts "a:c:f:o:p:s:" opt; do
     case $opt in
         a)
             ROOTFS_OVERLAYS="$ROOTFS_OVERLAYS $OPTARG"
@@ -66,6 +67,9 @@ while getopts "a:c:f:o:p:" opt; do
             # Append priorities to the master list
             cat "$OPTARG" >> "$SQUASHFS_PRIORITIES"
             ;;
+        s)
+            POST_PROCESSING_SCRIPT="$OPTARG"
+            ;;
         \?)
             echo "$SCRIPT_NAME: ERROR: Invalid option: -$OPTARG"
             usage
@@ -157,6 +161,12 @@ fi
 # Merge the Erlang/OTP release onto the base image
 "$NERVES_SYSTEM/scripts/merge-squashfs" "$NERVES_SDK_IMAGES/rootfs.squashfs" "$TMP_DIR/combined.squashfs" "$TMP_DIR/rootfs_overlay" "$SQUASHFS_PRIORITIES"
 
+# If a post processing script for the final rootfs was specified, run it
+if [ ! -z "${POST_PROCESSING_SCRIPT}" ]; then
+    echo "Running post-processing script: ${POST_PROCESSING_SCRIPT}"
+    ${POST_PROCESSING_SCRIPT} "$TMP_DIR/combined.squashfs";
+fi
+
 # Build the firmware image
 echo "Building $FW_FILENAME..."
 ROOTFS="$TMP_DIR/combined.squashfs" $FWUP -c -f "$FWUP_CONFIG" \