Provide msclock() as Milliseconds timer for performance measures (#231)

- don't use clock(). It has different functionalities in Windows and Linux
- move sleep functions to util.h

client/flash.c

@@ -12,8 +12,9 @@
 #include <string.h>
 #include <stdlib.h>
 #include <inttypes.h>
+#include <unistd.h>
 #include "proxmark3.h"
-#include "sleep.h"
+#include "util.h"
 #include "flash.h"
 #include "elf.h"
 #include "proxendian.h"