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/quefrency/Kconfig.defconfig

@@ -7,6 +7,9 @@ if SHIELD_QUEFRENCY_LEFT
 config ZMK_KEYBOARD_NAME
 	default "Quefrency Left"
 
+config ZMK_SPLIT_BLE_ROLE_CENTRAL
+	default y
+
 endif
 
 if SHIELD_QUEFRENCY_RIGHT
@@ -15,3 +18,10 @@ config ZMK_KEYBOARD_NAME
 	default "Quefrency Right"
 
 endif
+
+if SHIELD_QUEFRENCY_LEFT || SHIELD_QUEFRENCY_RIGHT
+
+config ZMK_SPLIT
+	default y
+
+endif