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/util/CMakeLists.txt b/programs/util/CMakeLists.txt
index cb14a3e..2a11212 100644
--- a/programs/util/CMakeLists.txt
+++ b/programs/util/CMakeLists.txt
@@ -1,5 +1,5 @@
 set(libs
-    mbedcrypto
+    ${mbedcrypto_target}
 )
 
 set(executables