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/x509/CMakeLists.txt b/programs/x509/CMakeLists.txt
index f7b5fe1..29cbeb8 100644
--- a/programs/x509/CMakeLists.txt
+++ b/programs/x509/CMakeLists.txt
@@ -1,5 +1,5 @@
 set(libs
-    mbedx509
+    ${mbedx509_target}
 )
 
 if(USE_PKCS11_HELPER_LIBRARY)
@@ -23,7 +23,7 @@
     target_link_libraries(${exe} ${libs})
 endforeach()
 
-target_link_libraries(cert_app mbedtls)
+target_link_libraries(cert_app ${mbedtls_target})
 
 install(TARGETS ${executables}
         DESTINATION "bin"