soc/integration: move cpu_interface retro-compatibility to litex/__init__

litex/__init__.py

@@ -1 +1,6 @@
+import sys
 from litex.tools.litex_client import RemoteClient
+
+# retro-compat 2019-09-29
+from litex.soc.integration import export
+sys.modules["litex.soc.integration.cpu_interface"] = export

litex/soc/integration/__init__.py

@@ -1,6 +1,2 @@
-
 from litex.soc.integration.soc_core import SoCCore
 from litex.soc.integration.soc_sdram import SoCSDRAM
-
-# FIXME: retro-compat 2019-09-29
-from litex.soc.integration import export as cpu_interface