Use temp file for du calculations

Storing the du output in a variable seemed convenient, but I didn't
realise just how big it could get especially with things like infra
images -- there's something like 100MiB of text being stored in a bash
variable here.

Convert this to work with a temporary file

Change-Id: I6a6d22c2142e0f199490c39cca8c94769e4b0232

bin/disk-image-create

@@ -338,26 +338,28 @@ mv $TMP_BUILD_DIR/mnt $TMP_BUILD_DIR/built
 # logs with du output below.
 xtrace=$(set +o | grep xtrace)
 
+# temp file for holding du output
+du_output=${TMP_BUILD_DIR}/du_output.tmp
+
 if [ -n "$DIB_IMAGE_SIZE" ]; then
     du_size=$(echo "$DIB_IMAGE_SIZE" | awk '{printf("%d\n",$1 * 1024 *1024)}')
 else
     set +o xtrace
     echo "Calculating image size (this may take a minute)..."
-    du_output=$(sudo du -a -c -x ${TMP_BUILD_DIR}/built)
+    sudo du -a -c -x ${TMP_BUILD_DIR}/built > ${du_output}
     # the last line is the total size from "-c".
     # scale this by 0.6 to create a slightly bigger image
-    du_size=$(echo "$du_output" | tail -n1 | cut -f1 | \
-                     awk '{print int($1 / 0.6)}')
+    du_size=$(tail -n1 ${du_output} | cut -f1 | awk '{print int($1 / 0.6)}')
     $xtrace
 fi
 
 if [[ "${DIB_SHOW_IMAGE_USAGE:-0}" != 0 ]]; then
     set +o xtrace
-    if [ -z "$du_output" ]; then
-        du_output=$(sudo du -a -c -x ${TMP_BUILD_DIR}/built)
+    if [ ! -f "$du_output" ]; then
+        sudo du -a -c -x ${TMP_BUILD_DIR}/built > ${du_output}
     fi
 
-    du_output_show="sort -nr |
+    du_output_show="sort -nr ${du_output} |
                      numfmt --to=iec-i --padding=7
                        --suffix=B --field=1 --from-unit=1024"
 
@@ -376,7 +378,7 @@ if [[ "${DIB_SHOW_IMAGE_USAGE:-0}" != 0 ]]; then
         echo "================="
     fi
 
-    eval ${du_output_show} <<< "$du_output"
+    eval ${du_output_show}
 
     echo
     echo "===== end image size report ====="
@@ -385,6 +387,8 @@ if [[ "${DIB_SHOW_IMAGE_USAGE:-0}" != 0 ]]; then
     $xtrace
 fi
 
+rm -f ${du_output}
+
 if [ "$FS_TYPE" = "ext4" ] ; then
   # Very conservative to handle images being resized a lot
   # We set journal size to 64M so our journal is large enough when we