[mbedtls] make debug logging independent from builtin (#9848)

Currently mbedtls debug logging can only be set for builtin
mbedtls. With an external mbedtls the log level does not get
configured. This commit removes the builtin requirement.

src/core/crypto/mbedtls.cpp

@@ -53,11 +53,11 @@ namespace Crypto {
 
 MbedTls::MbedTls(void)
 {
-#if OPENTHREAD_CONFIG_ENABLE_BUILTIN_MBEDTLS_MANAGEMENT
 #ifdef MBEDTLS_DEBUG_C
     // mbedTLS's debug level is almost the same as OpenThread's
     mbedtls_debug_set_threshold(OPENTHREAD_CONFIG_LOG_LEVEL);
 #endif
+#if OPENTHREAD_CONFIG_ENABLE_BUILTIN_MBEDTLS_MANAGEMENT
     mbedtls_platform_set_calloc_free(Heap::CAlloc, Heap::Free);
 #endif // OPENTHREAD_CONFIG_ENABLE_BUILTIN_MBEDTLS_MANAGEMENT
 }