[nrf noup] boards: nordic: thingy53: Add sysbuild Kconfig file

Adds a sysbuild Kconfig file which enables external flash when
partition manager is enabled

Signed-off-by: Jamie McCrae <[email protected]>
(cherry picked from commit fa07e94)

Author: nordicjm

boards/nordic/thingy53/Kconfig.sysbuild

@@ -0,0 +1,5 @@
+# Copyright (c) 2024 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+config PM_EXTERNAL_FLASH_MCUBOOT_SECONDARY
+	default y if BOOTLOADER_MCUBOOT