Add entropy safety switch.

Add a switch that turns entropy collecting off entirely, but enables
mbed TLS to run in an entirely unsafe mode. Enables to test mbed TLS
on platforms that don't have their entropy sources integrated yet.
diff --git a/scripts/config.pl b/scripts/config.pl
index a6dcfe7..ea77821 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -18,6 +18,7 @@
 #
 # Things that shouldn't be enabled with "full".
 #
+#   MBEDTLS_TEST_WO_ENTROPY
 #   MBEDTLS_DEPRECATED_REMOVED
 #   MBEDTLS_HAVE_SSE2
 #   MBEDTLS_PLATFORM_NO_STD_FUNCTIONS
@@ -69,6 +70,7 @@
 EOU
 
 my @excluded = qw(
+MBEDTLS_TEST_WO_ENTROPY
 MBEDTLS_DEPRECATED_REMOVED
 MBEDTLS_HAVE_SSE2
 MBEDTLS_PLATFORM_NO_STD_FUNCTIONS