avr: Switch to standard snprintf() to convert unsigned int to char

The original ultoa() function is widely supported by Arduino boards,
however some other boards with custom BSPs may not include this
function, as it is non-standard / non-ISO.

Switch to a more generally available snprintf() call to improve
compatibility. This change also provide a slight speed boost to
some boards (e.g. RP2040) that may have a bad implementation.

Signed-off-by: Ricky Cheung <[email protected]>

Author: Ricky Cheung

Arduino_Code/Arduino_Code.ino

@@ -123,7 +123,7 @@ uint32_t ducos1a() {
   uint32_t const maxNonce = job->difficulty * 100 + 1;
   char nonceStr[10 + 1];
   for (uint32_t nonce = 0; nonce < maxNonce; nonce++) {
-    ultoa(nonce, nonceStr, 10);
+    snprintf(nonceStr, 10, "%lu", nonce);
 
     uint8_t const * hash_bytes = duco_hash_try_nonce(&hash, nonceStr);
     if (memcmp(hash_bytes, target, SHA1_HASH_LEN) == 0) {