system: Simplify esp_derive_local_mac() function

Previously this function included a loop but would never run more
than one iteration of it.

components/esp_hw_support/include/esp_mac.h

@@ -116,11 +116,12 @@ esp_err_t esp_read_mac(uint8_t* mac, esp_mac_type_t type);
 /**
   * @brief Derive local MAC address from universal MAC address.
   *
-  * This function derives a local MAC address from an universal MAC address.
-  * A `definition of local vs universal MAC address can be found on Wikipedia
-  * <https://en.wikipedia.org/wiki/MAC_address#Universal_vs._local>`.
-  * In ESP32, universal MAC address is generated from base MAC address in EFUSE or other external storage.
-  * Local MAC address is derived from the universal MAC address.
+  * This function copies a universal MAC address and then sets the U/L bit
+  * (bit 0x2) in the first octet, creating a locally administered MAC address.
+  *
+  * It's not recommended that the universal MAC address argument is already a
+  * locally administered MAC address, but in this case the first octet is XORed
+  * with 0x4 in order to create a different locally administered MAC address.
   *
   * @param  local_mac  Derived local MAC address, length: 6 bytes.
   * @param  universal_mac  Source universal MAC address, length: 6 bytes.

components/esp_hw_support/mac_addr.c

@@ -117,21 +117,20 @@ esp_err_t esp_efuse_mac_get_default(uint8_t* mac)
 
 esp_err_t esp_derive_local_mac(uint8_t* local_mac, const uint8_t* universal_mac)
 {
-    uint8_t idx;
-
     if (local_mac == NULL || universal_mac == NULL) {
         ESP_LOGE(TAG, "mac address param is NULL");
         return ESP_ERR_INVALID_ARG;
     }
 
     memcpy(local_mac, universal_mac, 6);
-    for (idx = 0; idx < 64; idx++) {
-        local_mac[0] = universal_mac[0] | 0x02;
-        local_mac[0] ^= idx << 2;
 
-        if (memcmp(local_mac, universal_mac, 6)) {
-            break;
-        }
+    const unsigned UL_BIT = 0x2;
+    local_mac[0] |= UL_BIT;
+
+    if (local_mac[0] == universal_mac[0]) {
+        // universal_mac was already local, so flip this bit instead
+        // (this is kept to be compatible with the previous behaviour of this function)
+        local_mac[0] ^= 0x4;
     }
 
     return ESP_OK;