cmake: sysbuild: b0: Fix not configuring string for key

Fixes the key variable not being expanded for b0n image signing

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

cmake/sysbuild/b0_mcuboot_signing.cmake

@@ -1,4 +1,4 @@
-# Copyright (c) 2020-2023 Nordic Semiconductor ASA
+# Copyright (c) 2020-2024 Nordic Semiconductor ASA
 # SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 
 # This file includes extra build system logic that is enabled when
@@ -12,6 +12,7 @@
 
 function(ncs_secure_boot_mcuboot_sign application bin_files signed_targets prefix)
  set(keyfile "${SB_CONFIG_BOOT_SIGNATURE_KEY_FILE}")
+ string(CONFIGURE "${keyfile}" keyfile)
 
  # Find imgtool. Even though west is installed, imgtool might not be.
  # The user may also have a custom manifest which doesn't include