refactor(splits): Ability to override split role.

* Move central/peripheral setting to Kconfig.defconfig
  files to allow left/right .conf files to override
  central/peripheral role for splits.

app/boards/shields/lily58/Kconfig.defconfig

@@ -4,6 +4,9 @@ if SHIELD_LILY58_LEFT
 config ZMK_KEYBOARD_NAME
 	default "Lily58 Left"
 
+config ZMK_SPLIT_BLE_ROLE_CENTRAL
+	default y
+
 endif
 
 if SHIELD_LILY58_RIGHT
@@ -15,6 +18,9 @@ endif
 
 if SHIELD_LILY58_LEFT || SHIELD_LILY58_RIGHT
 
+config ZMK_SPLIT
+	default y
+
 if ZMK_DISPLAY
 
 config I2C
@@ -52,3 +58,4 @@ endchoice
 endif # LVGL
 
 endif
+