Add Build Timer

common.sh

Index: common.sh
diff --git a/common.sh b/common.sh
index 395310b43c78e775b05e0ce7bf88db440d1868fa..6d7a7f9173d6727dc4af34f3fb417a805b25e4aa 100644
--- a/common.sh
+++ b/common.sh
@@ -429,3 +429,15 @@ setup_symlinks_on_root() {
 
   sudo ln -s "${var_target}" "${dev_image_root}/var"
 }
+
+# Get current timestamp. Assumes common.sh runs at startup.
+start_time=$(date +%s)
+
+# Print time elsapsed since start_time.
+print_time_elapsed() {
+  end_time=$(date +%s)
+  elapsed_seconds="$(( $end_time - $start_time ))"
+  minutes="$(( $elapsed_seconds / 60 ))"
+  seconds="$(( $elapsed_seconds % 60 ))"
+  echo "Elapsed time: ${minutes}:${seconds}"
+}