fix(read_flash): flush transmit buffer less often to inrease throughput

Closes espressif/esptool#936

flasher_stub/stub_io.c

@@ -171,8 +171,15 @@ void stub_tx_one_char(char c)
 #endif // WITH_USB_OTG
   uart_tx_one_char(c);
 #if WITH_USB_JTAG_SERIAL
+  static unsigned short transferred_without_flush = 0;
   if (stub_uses_usb_jtag_serial()){
-    stub_tx_flush();
+    // Defer flushing until we have a (full - 1) packet or a end of packet (0xc0) byte to increase throughput.
+    // Note that deferring flushing until we have a full packet can cause hang-ups on some platforms.
+    ++transferred_without_flush;
+    if (c == '\xc0' || transferred_without_flush >= 63) {
+      stub_tx_flush();
+      transferred_without_flush = 0;
+    }
   }
 #endif // WITH_USB_JTAG_SERIAL
 }