Add option to test constant-flow with valgrind

Currently the new component in all.sh fails because
mbedtls_ssl_cf_memcpy_offset() is not actually constant flow - this is on
purpose to be able to verify that the new test works.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/scripts/config.py b/scripts/config.py
index 793e9df..37a2475 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -195,6 +195,7 @@
     'MBEDTLS_SHA512_NO_SHA384', # removes a feature
     'MBEDTLS_SSL_HW_RECORD_ACCEL', # build dependency (hook functions)
     'MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN', # build dependency (clang+memsan)
+    'MBEDTLS_TEST_CONSTANT_FLOW_VALGRIND', # build dependency (valgrind headers)
     'MBEDTLS_TEST_NULL_ENTROPY', # removes a feature
     'MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION', # influences the use of X.509 in TLS
     'MBEDTLS_ZLIB_SUPPORT', # build dependency (libz)