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).

rtl837x_common.h

@@ -173,6 +173,7 @@ void reset_chip(void);
 void memcpy(__xdata void * __xdata dst, __xdata const void * __xdata src, uint16_t len);
 void memcpyc(__xdata uint8_t *dst, __code uint8_t *src, uint16_t len);
 void memset(__xdata uint8_t *dst, __xdata uint8_t v, uint8_t len);
+int memcmp(__xdata const void *a, __xdata const void *b, uint16_t len);
 uint16_t strlen(__code const char *s);
 uint16_t strlen_x(__xdata const char *s);
 uint16_t strtox(__xdata uint8_t *dst, __code const char *s);