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/hid-flasher/flash.c

@@ -11,7 +11,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
-#include "sleep.h"
 #include "proxusb.h"
 #include "flash.h"
 #include "elf.h"