conda: mark the artiq-build output package as noarch, not toplevel.

This also changes `noarch: python` to `noarch: generic` since
this is semantically correct; the bitstream/firmware packages
contain no Python code.

Fixes #960.

conda/artiq-board/meta.yaml

@@ -6,7 +6,6 @@ source:
   git_url: ../..
 
 build:
-  noarch: python
   number: {{ environ["GIT_DESCRIBE_NUMBER"] }}
   string: py_{{ environ["GIT_DESCRIBE_NUMBER"] }}+git{{ environ["GIT_FULL_HASH"][:8] }}
   script_env:
@@ -16,6 +15,7 @@ build:
 
 outputs:
   - name: artiq-{{ environ["ARTIQ_TARGET"] }}-{{ environ["ARTIQ_VARIANT"] }}
+    noarch: generic
     files:
       - lib