Fix 3rdparty target names for custom config

Use the correct names qualified by MBEDTLS_TARGET_PREFIX.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/3rdparty/everest/CMakeLists.txt b/3rdparty/everest/CMakeLists.txt
index eefc151..e0e5ade 100644
--- a/3rdparty/everest/CMakeLists.txt
+++ b/3rdparty/everest/CMakeLists.txt
@@ -18,11 +18,11 @@
 # everest is not directly linked against any mbedtls targets
 # so does not inherit the compile definitions.
 if(MBEDTLS_CONFIG_FILE)
-    target_compile_definitions(everest
+    target_compile_definitions(${everest_target}
         PUBLIC MBEDTLS_CONFIG_FILE="${MBEDTLS_CONFIG_FILE}")
 endif()
 if(MBEDTLS_USER_CONFIG_FILE)
-    target_compile_definitions(everest
+    target_compile_definitions(${everest_target}
         PUBLIC MBEDTLS_USER_CONFIG_FILE="${MBEDTLS_USER_CONFIG_FILE}")
 endif()