Add pico_get_unique_board_id_string API (#281)

* Add pico_get_unique_board_id_string API

Add a new API to pico_unique which will turn the unique ID into a
canonical text string.

Use this API to update the USB device serial number in stdio_usb.

Supercedes #280

* Clean up -Wconversion=error issues

* 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_unique_id/include/pico/unique_id.h

@@ -57,6 +57,21 @@ typedef struct {
  */
 void pico_get_unique_board_id(pico_unique_board_id_t *id_out);
 
+/*! \brief Get unique ID in string format
+ *  \ingroup pico_unique_id
+ *
+ * Get the unique 64-bit device identifier which was retrieved from the
+ * external NOR flash device at boot, formatted as an ASCII hex string.
+ * Will always 0-terminate.
+ *
+ * On PICO_NO_FLASH builds the unique identifier is set to all 0xEE.
+ *
+ * \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, uint len);
+
+
 #ifdef __cplusplus
 }
 #endif

src/rp2_common/pico_unique_id/unique_id.c

@@ -26,3 +26,14 @@ static void __attribute__((constructor)) _retrieve_unique_id_on_boot(void) {
 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, uint len) {
+    assert(len > 0);
+    size_t i;
+    // Generate hex one nibble at a time
+    for (i = 0; (i < len - 1) && (i < PICO_UNIQUE_BOARD_ID_SIZE_BYTES * 2); i++) {
+        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;
+}