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/ssl/CMakeLists.txt b/programs/ssl/CMakeLists.txt
index 28fbfc5..ca0a6a4 100644
--- a/programs/ssl/CMakeLists.txt
+++ b/programs/ssl/CMakeLists.txt
@@ -2,7 +2,7 @@
 find_package(Threads)
 
 set(libs
-    mbedtls
+    ${mbedtls_target}
 )
 
 if(USE_PKCS11_HELPER_LIBRARY)