image.bbclass: support chaining compression (aka conversion) commands

It makes sense to use the compression mechanism also for conversion,
for example of a whole-disk image into .vdi (VirtualBox). That part
already works, like this:

   COMPRESSIONTYPES_append = " vdi"
   COMPRESS_CMD_vdi = "qemu-img convert -O vdi ${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.${type} ${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.${type}.vdi"
   IMAGE_DEPENDS_vdi = "qemu-native"

But then it also makes sense to allow compressing the resulting image,
which only works after enhancing the image.bbclass.

For example, suppose a custom image command produces "dsk" images. Then
it becomes possible to set
   IMAGE_FSTYPES = " dsk.xz dsk.vdi.xz"
and do_image_dsk will automatically produce the intermediate images,
convert to dsk.xz resp. dsk.vdi -> dsk.vdi.xz and delete all
intermediate images. Symlinks are also set correctly.

(From OE-Core rev: 588f14370372a66329b54606071175519ce88f1e)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/image.bbclass

@@ -303,6 +303,10 @@ python () {
                 basetype = type[:-len("." + ctype)]
                 break
 
+        if basetype != type:
+            # New base type itself might be generated by a conversion command.
+            basetype = _image_base_type(basetype)
+
         return basetype
 
     basetypes = {}
@@ -379,18 +383,35 @@ python () {
             bb.fatal("No IMAGE_CMD defined for IMAGE_FSTYPES entry '%s' - possibly invalid type name or missing support class" % t)
         cmds.append(localdata.expand("\tcd ${DEPLOY_DIR_IMAGE}"))
 
-        for bt in basetypes[t]:
+        rm_tmp_images = set()
+        def gen_conversion_cmds(bt):
             for ctype in ctypes:
                 if bt.endswith("." + ctype):
+                    type = bt[0:-len(ctype) - 1]
+                    # Create input image first.
+                    gen_conversion_cmds(type)
+                    localdata.setVar('type', type)
                     cmds.append("\t" + localdata.getVar("COMPRESS_CMD_" + ctype, True))
                     vardeps.add('COMPRESS_CMD_' + ctype)
-                    subimages.append(realt + "." + ctype)
+                    subimages.append(type + "." + ctype)
+                    if type not in alltypes:
+                        rm_tmp_images.add(localdata.expand("${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.${type}"))
 
+        for bt in basetypes[t]:
+            gen_conversion_cmds(bt)
+
+        localdata.setVar('type', realt)
         if realt not in alltypes:
-            cmds.append(localdata.expand("\trm ${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.${type}"))
+            rm_tmp_images.add(localdata.expand("${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.${type}"))
         else:
             subimages.append(realt)
 
+        # Clean up after applying all conversion commands. Some of them might
+        # use the same input, therefore we cannot delete sooner without applying
+        # some complex dependency analysis.
+        for image in rm_tmp_images:
+            cmds.append("\trm " + image)
+
         after = 'do_image'
         for dep in typedeps[t]:
             after += ' do_image_%s' % dep.replace("-", "_").replace(".", "_")