Address review comments, fix api typing

Use cleaner binary-to-hex conversion.  Update the length parameter to
use uint per the SDK standard .

src/rp2_common/pico_stdio_usb/stdio_usb_descriptors.c

@@ -125,8 +125,9 @@ const uint16_t *tud_descriptor_string_cb(uint8_t index, uint16_t langid) {
     static uint16_t desc_str[DESC_STR_MAX];
 
     // Assign the SN using the unique flash id
-    if (!usbd_serial_str[0])
+    if (!usbd_serial_str[0]) {
         pico_get_unique_board_id_string(usbd_serial_str, sizeof(usbd_serial_str));
+    }
 
     uint8_t len;
     if (index == 0) {

src/rp2_common/pico_unique_id/include/pico/unique_id.h

@@ -69,7 +69,7 @@ void pico_get_unique_board_id(pico_unique_board_id_t *id_out);
  * \param id_out a pointer to a char buffer of size len, to which the identifier will be written
  * \param len the size of id_out. For full serial, len >= 2 * PICO_UNIQUE_BOARD_ID_SIZE_BYTES + 1
  */
-void pico_get_unique_board_id_string(char *id_out, size_t len);
+void pico_get_unique_board_id_string(char *id_out, uint len);
 
 
 #ifdef __cplusplus

src/rp2_common/pico_unique_id/unique_id.c

@@ -27,14 +27,12 @@ void pico_get_unique_board_id(pico_unique_board_id_t *id_out) {
     *id_out = retrieved_id;
 }
 
-void pico_get_unique_board_id_string(char *id_out, size_t len) {
-    pico_unique_board_id_t temp = retrieved_id;
-    size_t i;
+void pico_get_unique_board_id_string(char *id_out, uint len) {
+    assert(len > 0);
+    int i;
     // Generate hex one nibble at a time
     for (i = 0; (i < len - 1) && (i < PICO_UNIQUE_BOARD_ID_SIZE_BYTES * 2); i++) {
-        size_t bi = i / 2;
-        int nibble = (temp.id[bi] >> 4) & 0x0F;
-        temp.id[bi] = (uint8_t)(temp.id[bi] << 4);
+        int nibble = (retrieved_id.id[i/2] >> (4 - 4 * (i&1))) & 0xf;
         id_out[i] = (char)(nibble < 10 ? nibble + '0' : nibble + 'A' - 10);
     }
     id_out[i] = 0;