Extensible split data sync (#11930)

* Extensible split data sync capability through transactions.

- Split common transport has been split up between the transport layer
  and data layer.
- Split "transactions" model used, with convergence between I2C and
  serial data definitions.
- Slave matrix "generation count" is used to determine if the full slave
  matrix needs to be retrieved.
- Encoders get the same "generation count" treatment.
- All other blocks of data are synchronised when a change is detected.
- All transmissions have a globally-configurable deadline before a
  transmission is forced (`FORCED_SYNC_THROTTLE_MS`, default 100ms).
- Added atomicity for all core-synced data, preventing partial updates
- Added retries to AVR i2c_master's i2c_start, to minimise the number of
  failed transactions when interrupts are disabled on the slave due to
  atomicity checks.
- Some keyboards have had slight modifications made in order to ensure
  that they still build due to firmware size restrictions.

* Fixup LED_MATRIX compile.

* Parameterise ERROR_DISCONNECT_COUNT.

tmk_core/common/host.c

@@ -17,6 +17,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <stdint.h>
 //#include <avr/interrupt.h>
+#include "keyboard.h"
 #include "keycode.h"
 #include "host.h"
 #include "util.h"
@@ -35,15 +36,20 @@ void host_set_driver(host_driver_t *d) { driver = d; }
 
 host_driver_t *host_get_driver(void) { return driver; }
 
+#ifdef SPLIT_KEYBOARD
+uint8_t split_led_state = 0;
+void    set_split_host_keyboard_leds(uint8_t led_state) { split_led_state = led_state; }
+#endif
+
 uint8_t host_keyboard_leds(void) {
+#ifdef SPLIT_KEYBOARD
+    if (!is_keyboard_master()) return split_led_state;
+#endif
     if (!driver) return 0;
     return (*driver->keyboard_leds)();
 }
 
-led_t host_keyboard_led_state(void) {
-    if (!driver) return (led_t){0};
-    return (led_t)((*driver->keyboard_leds)());
-}
+led_t host_keyboard_led_state(void) { return (led_t)host_keyboard_leds(); }
 
 /* send report */
 void host_keyboard_send(report_keyboard_t *report) {