sd-netlink: introduce netlink_message_{read,append}_hw_addr

Hardware addresses come in various shapes and sizes, these new functions
and accomapying data structures account for that instead of hard-coding
a hardware address to the 6 bytes of an ethernet MAC.

src/basic/ether-addr-util.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include <errno.h>
+#include <inttypes.h>
 #include <net/ethernet.h>
 #include <stdio.h>
 #include <sys/types.h>
@@ -9,6 +10,20 @@
 #include "macro.h"
 #include "string-util.h"
 
+char* hw_addr_to_string(const hw_addr_data *addr, char buffer[HW_ADDR_TO_STRING_MAX]) {
+        assert(addr);
+        assert(buffer);
+        assert(addr->length <= HW_ADDR_MAX_SIZE);
+
+        for (size_t i = 0; i < addr->length; i++) {
+                sprintf(&buffer[3*i], "%02"PRIx8, addr->addr.bytes[i]);
+                if (i < addr->length - 1)
+                        buffer[3*i + 2] = ':';
+        }
+
+        return buffer;
+}
+
 char* ether_addr_to_string(const struct ether_addr *addr, char buffer[ETHER_ADDR_TO_STRING_MAX]) {
         assert(addr);
         assert(buffer);