Add ability to add output marker via external signal

For example, a script to flash the chip can use this to mark the
output so that it's clear when the target was actually reset.

itm-decode.py

@@ -13,6 +13,20 @@ color_lookup = { "red": 31, "green": 32, "cyan": 36, "yellow": 33 }
 def color(name, text):
     return "\033[%dm%s\033[0m" % (color_lookup[name], text);
 
+try:
+    # On Unix systems, print an additional marker in the output stream
+    # when SIGUSR1 or SIGUSR2 is received.  This is just to help clarify
+    # output when the chip is externally reset, etc.
+    import signal
+    def sigusr1_handler(signum, frame):
+        print(color("yellow", "--- mark ---"))
+    def sigusr2_handler(signum, frame):
+        print(color("yellow", "--- reset ---"))
+    signal.signal(signal.SIGUSR1, sigusr1_handler)
+    signal.signal(signal.SIGUSR2, sigusr2_handler)
+except AttributeError as e:
+    pass
+
 class ResyncException(Exception):
     pass