Allow "deleting" url/path of repo in override

Make "null" an allowed type for the `url` and `path` properties of a
repository.  This allows "deleting" them in an override kas-file to e.g.
point a repository at a local directory instead of a git remote:

"Original" file:

    repos:
      meta-foo:
        url: https://example.org/foo.git
        refspec: badc0ffee

Override:

    repos:
      meta-foo:
        url:
        path: /path/to/local/foo

Without the explicit "nulling" of the URL, kas would start messing with
the repo in /path/to/local/foo (changing remote URL, switching to a
different rev, etc.).  With this change, we can instead force kas to use
the recipes in that directory untouched.

Signed-off-by: Harald Seiler <hws@denx.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/format-changelog.rst

@@ -111,3 +111,13 @@ Added
 
 - ``menu_configuration`` key stores the selections done via ``kas menu`` in a
   configuration file. It is only evaluated by that plugin.
+
+Version 12
+----------
+
+Added
+~~~~~
+
+- For repositories, ``url`` and ``path`` can now be overridden with a
+  null-value to switch between version-controlled repositories and unversioned
+  local folders.