config: Implemented multi-target support

With this patch support for building multiple targets in parallel with
bitbake is now supported.

Changes:

  - Its now possible to state a list of targets under the target key in
    the configuration file.

    Example:

        target:
          - product-image
          - product-update-image

  - Its now possible to define multiple targets in the kas command line.

    Example:

        $ kas build --target product-image --target product-update-image \
          kas.yml

  - Its now possible to define multiple targets via the environment:

    Example:

        $ export KAS_TARGET="product-image product-update-image"
        $ kas build kas.yml

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

kas/config.py

@@ -209,13 +209,19 @@ class Config:
             repo_dict[repo] = rep
         return repo_dict
 
-    def get_bitbake_target(self):
+    def get_bitbake_targets(self):
         """
-            Return the bitbake target
+            Returns a list of bitbake targets
         """
-        return os.environ.get('KAS_TARGET',
-                              self._config.get('target',
-                                               'core-image-minimal'))
+        environ_targets = [i
+                           for i in os.environ.get('KAS_TARGET', '').split()
+                           if i]
+        if environ_targets:
+            return environ_targets
+        target = self._config.get('target', 'core-image-minimal')
+        if isinstance(target, str):
+            return [target]
+        return target
 
     def get_bitbake_task(self):
         """