unix-time-to-fmt.py: fix crash without argument

Print "unknown" to stdout when unix-time-to-fmt.py was called without
an argument, instead of printing a Python stack trace to stderr. It gets
called that way when building outside of a git folder.

Change-Id: I3ba27cc23543e47cc8a1e494c35cc33f1dc8f297

build/unix-time-to-fmt.py

@@ -11,6 +11,10 @@ Default is '%Y-%b-%d' --> 2016-Jan-01
 
 import sys, time
 
+if len(sys.argv) == 1:
+    print("unknown")
+    exit(0)
+
 fmt = '%Y-%b-%d'
 if len(sys.argv) > 2:
   fmt = sys.argv[2]