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/scripts/config.pl b/scripts/config.pl
index b667905..05cc52e 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -107,6 +107,7 @@
 MBEDTLS_MEMORY_BUFFER_ALLOC_C
 MBEDTLS_PLATFORM_TIME_ALT
 MBEDTLS_PLATFORM_FPRINTF_ALT
+MBEDTLS_PSA_CRYPTO_SE_C
 MBEDTLS_PSA_CRYPTO_STORAGE_C
 MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
 MBEDTLS_PSA_ITS_FILE_C