Build: Add MBEDTLS_TARGET_PREFIX

Allows required targets to have prefixes added to them, so that external
projects can avoid target names clashing.

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/programs/psa/CMakeLists.txt b/programs/psa/CMakeLists.txt
index 5cbcf71..23e85fe 100644
--- a/programs/psa/CMakeLists.txt
+++ b/programs/psa/CMakeLists.txt
@@ -6,7 +6,7 @@
 
 foreach(exe IN LISTS executables)
     add_executable(${exe} ${exe}.c $<TARGET_OBJECTS:mbedtls_test>)
-    target_link_libraries(${exe} mbedcrypto)
+    target_link_libraries(${exe} ${mbedcrypto_target})
     target_include_directories(${exe} PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../../tests/include)
 endforeach()