feat(cli): add mac command to set the management MAC from the config

Add a `mac <aa:bb:cc:dd:ee:ff>` console command (and a bare `mac` to show
the current address) that overrides the management MAC at runtime. The
address is validated (unicast, globally administered, non-zero OUI) and
the static L2 management entry is re-registered so the change applies
without a reboot. Because it is a normal command, a line in the startup
config persists it across boots: execute_config() runs it before the final
static L2 entry and the derived hostname are set up.

Also point the keepLink KP-9000-9XHML-X V3.1 machine definition at the
factory MAC area (0x1FC000, same location other machines use) so a valid
factory MAC is picked up at boot instead of falling back to a locally
administered address.

The MAC parsing/printing lives in parse_mac()/print_mac() helpers next to
parse_ip()/print_ip() so future commands can reuse them, and a memcmp()
helper joins the project's own memcpy()/memset() (no libc is linked).

machine.c

@@ -247,6 +247,7 @@ __code const struct machine machine = {
 	.led_mux = {0x00, 0x01, 0x04, 0x05, 0x08, 0x09, 0x0c, 0x09, 0x0d, 0x10,
 				0x11, 0x0e, 0x14, 0x11, 0x12, 0x15, 0x15, 0x16, 0x18, 0x19,
 				0x1a, 0x19, 0x1d, 0x1e, 0x1c, 0x1d, 0x20, 0x21},
+	.mac_flash_offset = 0x1FC000,
 };
 
 #elif defined MACHINE_SWGT024_V2_0_MANAGED