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/random/CMakeLists.txt b/programs/random/CMakeLists.txt
index 95acb7e..8df8365 100644
--- a/programs/random/CMakeLists.txt
+++ b/programs/random/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})
 endforeach()
 
 install(TARGETS ${executables}