Extend the checksum files generation procedure

The usage of the DIB_CHECKSUM variable is extended to have an
ability generate the only one checksum file, for example only 'sha256'
(by setting an environment variable DIB_CHECKSUM='sha256'), and to
retain the backward compatibility (DIB_CHECKSUM=1 will generate
both 'sha256' and 'md5' supported at this moment). As an additional
feature we have the simple way to completely deprecate 'md5' later,
and add new methods, for example, 'sha512' etc.

Change-Id: I2dd1c60e3bfd9c823a7382b1390b1d40c52a5c97
Signed-off-by: Maksim Malchuk <maksim.malchuk@gmail.com>

diskimage_builder/lib/common-functions

@@ -65,7 +65,7 @@ function finish_image () {
     fi
 
     mv $OUT_IMAGE_PATH $1
-    if [ "$DIB_CHECKSUM" == "1" ]; then
+    if [[ -n "$DIB_CHECKSUM" && "$DIB_CHECKSUM" != "0" ]]; then
 
       # NOTE(pabelanger): Read image into memory once and generate
       # both checksum files.
@@ -74,8 +74,9 @@ function finish_image () {
       # no tty).  Waiting for just these processes is a bit of hacky
       # workaround ...
       declare -a wait_for
-      md5sum $1 > $1.md5 & wait_for+=($!)
-      sha256sum $1 > $1.sha256 & wait_for+=($!)
+      [[ "$DIB_CHECKSUM" == "1" ]] && DIB_CHECKSUM="md5,sha256"
+      [[ "$DIB_CHECKSUM" == *md5* ]] && md5sum $1 > $1.md5 & wait_for+=($!)
+      [[ "$DIB_CHECKSUM" == *sha256* ]] && sha256sum $1 > $1.sha256 & wait_for+=($!)
       wait "${wait_for[@]}"
     fi
     echo "Image file $1 created..."

releasenotes/notes/dib-checksum-3d0d9af8778176be.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    The usage of the ``DIB_CHECKSUM`` variable is extended. Now to be more
+    specific the variable can be set to the ``sha256`` or ``md5`` value to
+    generate only one checksum file. There also can be provided a
+    comma-separated list of the values, but only ``sha256`` and ``md5``
+    supported. For backward compatibility using ``DIB_CHECKSUM=1`` or option
+    ``--checksum`` in the command line still can be used to generate all
+    supported checksums.