feat(l0dables): Return to menu when main() exits

After main() returns, don't enter a busy loop but call epic_exit() to
signal to epicardium that the l0dable has finished.  The exit code will
be the return value from main().

Signed-off-by: Rahix <rahix@rahix.de>

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)
 {

l0dables/lib/crt.s

@@ -157,12 +157,9 @@ Reset_Handler:
 		ldr r0, =main
 		blx r0
 
-		/*
-		 * C code done, spin forever.
-		 * TODO(q3k): let epicardium know we're done.
-		 */
-.spin:
-		bl .spin
+		/* C code done, return to menu.  Return code is what main() returned. */
+		ldr r4, =epic_exit
+		blx r4
 
 		/*
 		 * Used by __libc_init_array.