Unique USB serial number from the MCU ID (issue #85)

All units reported the ChibiOS kernel version "400" as USB serial number,
so Windows could not tell two tinySAs apart: with two of them on the same
VID/PID only one enumerated, and a unit could get another unit's COM port.

The serial number is now 12 hex digits made from the MCU unique ID the way
the STM32F303 DFU bootloader makes its serial number (UID0 + UID2, then the
upper half of UID2): a tinySA Ultra has the same serial as COM port and in
DFU mode. It is filled at every USB reset, before the host asks for it.
Windows assigns every unit a new COM port once.

Tested: ZS407 enumerates as 1781:0A44\205D346A2039 (its DFU serial), the
tinySA BASIC as 0483:5740\2056325F2039, both work side by side.
BASIC about +80 B flash, +26 B RAM.

usbcfg.c

@@ -248,74 +248,28 @@ static const uint8_t vcom_string2[] = {
 };
 #endif
 
-#ifdef TINYSA4
 /*
- * Serial Number string. VERSION = 'tinySA4_v1.3-nnn-gxxxxxxx'
- *                                  01234567890123456789012
- * skip last two 'xx' char due to  'tinySA4_v1.3-n-gxxxxxxx'
+ * Serial Number string: 12 hex digits made from the MCU unique ID the way the STM32F303 DFU
+ * bootloader makes its serial number (UID0 + UID2, then the upper half of UID2), so every tinySA
+ * has its own. It was the kernel version "400" on all of them: Windows could not tell two units
+ * apart and gave one unit's COM port to the other. Filled at every USB reset, before it is requested.
  */
-static const uint8_t vcom_string3[] =
+static uint8_t vcom_string3[2 + 2 * 12];       // the upper bytes of the UTF-16 characters stay 0
+
+static void fill_serial_string(void)
 {
-#if 1
- USB_DESC_BYTE(8),                    /* bLength.                         */
- USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
- '0' + CH_KERNEL_MAJOR, 0,
- '0' + CH_KERNEL_MINOR, 0,
- '0' + CH_KERNEL_PATCH, 0
-#else
-  USB_DESC_BYTE(32),                    /* bLength.                         */
-  USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
-  VERSION[8], 0,  /* 'v' */
-  VERSION[9], 0,  /* '1' */
-  VERSION[10], 0, /* '.' */
-  VERSION[11], 0, /* '3' */
-  VERSION[12], 0, /* '-' */
-  VERSION[13], 0, /* 'n' */
-  VERSION[14], 0, /* 'n' */
-  VERSION[15], 0, /* 'n' */
-  VERSION[16], 0, /* '-' */
-  VERSION[17], 0, /* 'g' */
-  VERSION[18], 0, /* 'x' */
-  VERSION[19], 0, /* 'x' */
-  VERSION[20], 0, /* 'x' */
-  VERSION[21], 0, /* 'x' */
-  VERSION[22], 0, /* 'x' */
-#endif
-};
-#else
-/*
- * Serial Number string. VERSION = 'tinySA_v1.3-nnn-gxxxxxxx'
- *                                  0123456789012345678901
- * skip last two 'xx' char due to  'tinySA_v1.3-n-gxxxxxxx'
- */
-static const uint8_t vcom_string3[] = {
-#if 1
- USB_DESC_BYTE(8),                    /* bLength.                         */
- USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
- '0' + CH_KERNEL_MAJOR, 0,
- '0' + CH_KERNEL_MINOR, 0,
- '0' + CH_KERNEL_PATCH, 0
-#else
- USB_DESC_BYTE(32),                    /* bLength.                         */
-  USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
-  VERSION[7], 0,  /* 'v' */
-  VERSION[8], 0,  /* '1' */
-  VERSION[9], 0,  /* '.' */
-  VERSION[10], 0, /* '3' */
-  VERSION[11], 0, /* '-' */
-  VERSION[12], 0, /* 'n' */
-  VERSION[13], 0, /* 'n' */
-  VERSION[14], 0, /* 'n' */
-  VERSION[15], 0, /* '-' */
-  VERSION[16], 0, /* 'g' */
-  VERSION[17], 0, /* 'x' */
-  VERSION[18], 0, /* 'x' */
-  VERSION[19], 0, /* 'x' */
-  VERSION[20], 0, /* 'x' */
-  VERSION[21], 0, /* 'x' */
-#endif
-};
-#endif
+  const uint32_t *uid = (const uint32_t *)UID_BASE;
+  uint32_t v = uid[0] + uid[2];
+  vcom_string3[0] = sizeof vcom_string3;
+  vcom_string3[1] = USB_DESCRIPTOR_STRING;
+  for (int i = 2; i < (int)sizeof vcom_string3; i += 2) {
+    if (i == 2 + 2 * 8)                        // 8 digits done: then the upper half of UID2
+      v = uid[2];
+    uint8_t c = v >> 28;
+    v <<= 4;
+    vcom_string3[i] = c + (c < 10 ? '0' : 'A' - 10);
+  }
+}
 
 /*
  * Strings wrappers array.
@@ -406,6 +360,7 @@ static void usb_event(USBDriver *usbp, usbevent_t event) {
 
   switch (event) {
   case USB_EVENT_RESET:
+    fill_serial_string();
 #ifdef __USE_USB_MSC__
     {
       /* usb_lld_reset() has just allocated the EP0 buffers.*/