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).

include/mbedtls/config.h

@@ -1709,6 +1709,19 @@
  */
 #define MBEDTLS_PSA_CRYPTO_C
 
+/**
+ * \def MBEDTLS_PSA_CRYPTO_SE_C
+ *
+ * Enable secure element support in the Platform Security Architecture
+ * cryptography API.
+ *
+ * Module:  library/psa_crypto_se.c
+ *
+ * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_PSA_CRYPTO_STORAGE_C
+ *
+ */
+#define MBEDTLS_PSA_CRYPTO_SE_C
+
 /**
  * \def MBEDTLS_PSA_CRYPTO_STORAGE_C
  *