Allow overwriting old images

The two duplicated functions, save_image and finish_image will move
an existing image out of the way if it exists, but it isn't
configurable. Check an environment variable is 0 before doing so.

Switch save_image to just calling finish_image, rather than
duplicating its code exactly.

Change-Id: I26a5a8fa4b6e853c9440bffab195b0bc3728be40

README.md

@@ -74,6 +74,11 @@ It's a colon (:) separated path list, and it will work in a first path/element f
 first served approach. The included elements tree is used when no path is supplied,
 and is added to the end of the path if a path is supplied.
 
+By default, the image building scripts will not overwrite existing disk images,
+allowing you to compare the newly built image with the existing one. To change
+that behaviour, set the variable OVERWRITE\_OLD\_IMAGE to any value that isn't
+0.
+
 Requirements
 ============
 

lib/common-functions

@@ -43,9 +43,9 @@ function mk_build_dir () {
 }
 
 function finish_image () {
-    if [ -f $1 ]; then
+    if [ -f $1 -a ${OVERWRITE_OLD_IMAGE:-0} -eq 0 ]; then
       old_image="${1%.*}"-$(date +%Y.%m.%d-%H.%M.%S).${1##*.}
-      echo "Old image is found. Renaming it to $old_image"
+      echo "Old image found. Renaming it to $old_image"
       mv "$1" "$old_image"
     fi
 
@@ -57,17 +57,7 @@ function finish_image () {
 }
 
 function save_image () {
-    if [ -f  $1 ] ; then
-      old_image="${1%.*}"-$(date +%Y.%m.%d-%H.%M.%S).${1##*.}
-      echo "Old image is found. Renaming it to $old_image"
-      mv "$1" "$old_image"
-    fi
-
-    cp $TMP_IMAGE_PATH $1
-    cleanup_dirs
-    # All done!
-    trap EXIT
-    echo "Image file $1 created..."
+    finish_image $1
 }
 
 function element_pre_check() {