bitbake: toaster: Fix IMAGE_INSTALL issues with _append vs :append

After the override syntax change, toaster isn't working correctly. This
is because it uses IMAGE_INSTALL_append instead of IMAGE_INSTALL:append.

This tweaks the code accordingly to fix this.

I have a suspicion that exiting toaster databases may struggle with
the change and there are some migration steps missing for the whole
overrides syntax change step.

(Bitbake rev: befc93c3e03a9b1a2b630cd5badcda756fe6cd28)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/toaster/orm/fixtures/settings.xml

@@ -19,7 +19,7 @@
     <field type="CharField" name="value">${TOPDIR}/../sstate-cache</field>
   </object>
   <object model="orm.toastersetting" pk="6">
-    <field type="CharField" name="name">DEFCONF_IMAGE_INSTALL_append</field>
+    <field type="CharField" name="name">DEFCONF_IMAGE_INSTALL:append</field>
     <field type="CharField" name="value"></field>
   </object>
   <object model="orm.toastersetting" pk="7">

bitbake/lib/toaster/orm/models.py

@@ -1717,7 +1717,7 @@ class CustomImageRecipe(Recipe):
 
     def generate_recipe_file_contents(self):
         """Generate the contents for the recipe file."""
-        # If we have no excluded packages we only need to _append
+        # If we have no excluded packages we only need to :append
         if self.excludes_set.count() == 0:
             packages_conf = "IMAGE_INSTALL:append = \" "