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/psa_crypto_se.c b/library/psa_crypto_se.c
index 32142eb..814c6a0 100644
--- a/library/psa_crypto_se.c
+++ b/library/psa_crypto_se.c
@@ -25,7 +25,7 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
-#if defined(MBEDTLS_PSA_CRYPTO_C)
+#if defined(MBEDTLS_PSA_CRYPTO_SE_C)
 
 #include <string.h>
 
@@ -76,4 +76,4 @@
     memset( driver_table, 0, sizeof( driver_table ) );
 }
 
-#endif /* MBEDTLS_PSA_CRYPTO_C */
+#endif /* MBEDTLS_PSA_CRYPTO_SE_C */