Clean exit of serialdump when hitting Ctrl-C

Ctrl-C is the best way to close serialdump after using make login.
With a clean exit it will signal to make that the make run was successful.

tools/sky/serialdump.c

@@ -8,6 +8,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <time.h>
+#include <signal.h>
 
 #define BAUDRATE B115200
 #define BAUDRATE_S "115200"
@@ -82,9 +83,17 @@ print_hex_line(char *prefix, unsigned char *outbuf, int index)
   }
 }
 
+static void
+intHandler(int sig)
+{
+  exit(0);
+}
+
 int
 main(int argc, char **argv)
 {
+  signal(SIGINT, intHandler);
+
   struct termios options;
   fd_set mask, smask;
   int fd;