extract-utils: Add prepare_firmware function

Allows devices to run required actions before attempting
to extract firmware.
Example: motorola star images & pixel fbpk images.

Change-Id: I105c3a17c10f63f0ad72966354aed6baed695d7c

extract_utils.sh

@@ -1877,6 +1877,13 @@ function extract_carriersettings() {
     python3 "$CARRIERSETTINGS_EXTRACTOR" -i "$SRC" -v "$CARRIERSETTINGS_OUTPUT_DIR"
 }
 
+#
+# To be overridden by device-level extract-files.sh
+#
+function prepare_firmware() {
+    :
+}
+
 #
 # extract_firmware:
 #
@@ -1907,6 +1914,8 @@ function extract_firmware() {
 
     echo "Extracting $COUNT files in $1 from $SRC:"
 
+    prepare_firmware
+
     for (( i=1; i<COUNT+1; i++ )); do
         local SRC_FILE=$(src_file "${FILELIST[$i-1]}")
         local DST_FILE=$(target_file "${FILELIST[$i-1]}")