Build: Tweak the SFN/IPC backend setting

Since by default TF-M is built with level 1 isolation, include SFN
backend config by default.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I415fa9f1ecd5ca2087400152787e6a2ae1624c23
diff --git a/config/tfm_ipc_config_default.cmake b/config/tfm_ipc_config_default.cmake
index 29dd236..4d7b026 100644
--- a/config/tfm_ipc_config_default.cmake
+++ b/config/tfm_ipc_config_default.cmake
@@ -9,6 +9,7 @@
 
 ############################ Partitions ########################################
 
+set(CONFIG_TFM_SPM_BACKEND      "IPC"       CACHE STRING    "The SPM backend [IPC, SFN]")
 set(CONFIG_TFM_SPM_BACKEND_IPC  ON)
 set(CONFIG_TFM_SPM_BACKEND_SFN  OFF)
 set(CONFIG_TFM_PARTITION_META   ON)