implements patch support for repos

With this commit its now possible to patch 3rd party repos before bitbake is started.

Example:

This is our repo:
  .
  ├── kas.yml
  ├── first-patch.patch
  └── quilt-patches
      ├── second-patch.patch
      ├── third-patch.patch
      └── series

Content of kas.yml:
  header:
    version: 8

  repos:
    my:
    third-party:
      url: "git://example.com/third-party.git"
      refspec: "35adf4...34"
      patches:
        01-first:
          repo: my
          path: "first-patch.patch"
        02-second:
          repo: my
          path: "quilt-patches"

Currently only 'git' repositories can be patched.

Signed-off-by: Claudius Heine <ch@denx.de>

docs/format-changelog.rst

@@ -66,3 +66,12 @@ Added
 
 - ``type`` property to ``repos`` to be able to express which version control
   system to use.
+
+Version 8
+---------
+
+Added
+~~~~~
+
+- ``patches`` property to ``repos`` to be able to apply additional patches to
+  the repo.

docs/userguide.rst

@@ -352,6 +352,22 @@ Configuration reference
           '0', 'false']``. This way it is possible to overwrite the inclusion
           of a layer in latter loaded configuration files.
 
+    * ``patches``: dict [optional]
+        Contains the patches that should be applied to this repo before it is
+        used.
+
+      * ``<patches-id>``: dict [optional]
+          One entry in patches with its specific and unique id. All available
+          patch entries are applied in the order of their sorted
+          ``<patches-id>``.
+
+        * ``repo``: string [required]
+            The identifier of the repo where the path of this entry is relative
+            to.
+
+        * ``path``: string [required]
+            The path to one patch file or a quilt formatted patchset directory.
+
 * ``bblayers_conf_header``: dict [optional]
     This contains strings that should be added to the ``bblayers.conf`` before
     any layers are included.