Add dlogf printf formatting, ipv4::to_string helper, clean up string building

firmware/include/ipv4.h

@@ -1,8 +1,10 @@
 #pragma once
 #include <array>
 #include <cstdint>
+#include <cstdio>
 #include <functional>
 #include <span>
+#include <string>
 #include "eth.h"
 #include "span_writer.h"
 
@@ -10,6 +12,12 @@ namespace ipv4 {
 
 using ip4_addr = std::array<uint8_t, 4>;
 
+inline std::string to_string(const ip4_addr& ip) {
+    char buf[16];
+    snprintf(buf, sizeof(buf), "%u.%u.%u.%u", ip[0], ip[1], ip[2], ip[3]);
+    return buf;
+}
+
 struct __attribute__((packed)) header {
     eth::header eth;
     uint8_t ver_ihl;