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/sofle/Kconfig.defconfig

@@ -5,6 +5,10 @@ if SHIELD_SOFLE_LEFT
 
 config ZMK_KEYBOARD_NAME
 	default "Sofle Left"
+
+config ZMK_SPLIT_BLE_ROLE_CENTRAL
+	default y
+	
 endif
 
 if SHIELD_SOFLE_RIGHT
@@ -15,6 +19,9 @@ endif
 
 if SHIELD_SOFLE_LEFT || SHIELD_SOFLE_RIGHT
 
+config ZMK_SPLIT
+	default y
+
 if ZMK_DISPLAY
 
 config I2C