A few improvements of the l0dable runtime

I was looking at the l0dable CRT and found a few things which could be improved:

1. After main() exits, don't busy spin but return from the l0dable with epic_exit().
2. In case of an exception, don't lock up the device but also attempt returning with epic_exit(). Without this, the only way to recover would be a full reboot of the card10.
3. Remove some bloat by making all unused interrupt handlers weak references to the default handler.

More details can be found in the individual commit messages.

l0dables/blinky/main.c

@@ -24,6 +24,9 @@ void fade()
 
 /*
  * main() is called when l0dable is loaded and executed.
+ *
+ * When main() returns, card10 goes back to the menu.  A non-zero return value
+ * can be used to signal some kind of failure during l0dable execution.
  */
 int main(void)
 {