app: Allow overriding PSA_API_TEST_TARGET via Cmake

To allow more flexibility in psa-arch-tests wrt platform naming

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
Change-Id: I281f88fe45e1219a18286f7a7b525ad63f6be6b6
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index 034b64a..1a5b836 100644
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -24,7 +24,9 @@
         set(SUITE ${TEST_PSA_API})
     endif()
 
-    string(REGEX REPLACE ".*/" "" PSA_API_TEST_TARGET ${TFM_PLATFORM})
+    if (NOT DEFINED PSA_API_TEST_TARGET)
+        string(REGEX REPLACE ".*/" "" PSA_API_TEST_TARGET ${TFM_PLATFORM})
+    endif()
 
     if(NOT TARGET)
         if (NOT "${TEST_PSA_API}" STREQUAL "IPC")