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/ChangeLog.d/fix-3rdparty-target-prefix.txt b/ChangeLog.d/fix-3rdparty-target-prefix.txt
new file mode 100644
index 0000000..db8ed07
--- /dev/null
+++ b/ChangeLog.d/fix-3rdparty-target-prefix.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fix accidental omission of MBEDTLS_TARGET_PREFIX in 3rdparty modules
+     in CMake.