yocto-bsp: set branches_base for list_property_values()

yocto_bsp_list_property_values() is missing the context it needs to
properly filter choicelists, so add it to the context object.

Fixes [YOCTO #3233]

(From meta-yocto rev: 064b15f76c5b52899f4c3fdef06412c3063062a5)

(From meta-yocto rev: 601b6227908f3dd7972ad62c53d1041f4429aeb2)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/bsp/engine.py

@@ -1445,6 +1445,10 @@ def yocto_bsp_list_property_values(arch, property, scripts_path, properties_file
             gen_fn = input_line.props["gen"]
             if nested_properties:
                 context["filename"] = nested_properties[0]
+                try:
+                    context["branches_base"] = input_line.props["branches_base"]
+                except KeyError:
+                    context["branches_base"] = None
             values_list = input_line.gen_choices_list(context, False)
         except KeyError:
             for choice in input_line.choices: