platform/Manifest.py: skip if no target defined

Allow support of generic platforms

platform/Manifest.py

-if target=="altera":
+import logging
+
+if 'target' not in globals():
+        logging.info("'target' is not defined, no platform selected")
+elif target=="altera":
 	modules = {"local" : "altera"}
 elif target=="xilinx":
 	modules = {"local" : "xilinx"}