Gate secure element support by a separate config option

Secure element support has its own source file, and in addition
requires many hooks in other files. This is a nontrivial amount of
code, so make it optional (but default on).
diff --git a/library/version_features.c b/library/version_features.c
index 4f1da6a..5701598 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -555,6 +555,9 @@
 #if defined(MBEDTLS_PSA_CRYPTO_C)
     "MBEDTLS_PSA_CRYPTO_C",
 #endif /* MBEDTLS_PSA_CRYPTO_C */
+#if defined(MBEDTLS_PSA_CRYPTO_SE_C)
+    "MBEDTLS_PSA_CRYPTO_SE_C",
+#endif /* MBEDTLS_PSA_CRYPTO_SE_C */
 #if defined(MBEDTLS_PSA_CRYPTO_STORAGE_C)
     "MBEDTLS_PSA_CRYPTO_STORAGE_C",
 #endif /* MBEDTLS_PSA_CRYPTO_STORAGE_C */