Add config.h option MBEDTLS_ECP_NO_INTERNAL_RNG

No effect so far, except on dependency checking, as the feature it's meant to
disable isn't implemented yet (so the descriptions in config.h and the
ChangeLog entry are anticipation for now).

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 93de091..0aef0d0 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -140,6 +140,14 @@
 #error "MBEDTLS_ECP_C defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_ECP_C) && !(            \
+    defined(MBEDTLS_ECP_ALT) ||             \
+    defined(MBEDTLS_CTR_DRBG_C) ||          \
+    defined(MBEDTLS_HMAC_DRBG_C) ||         \
+    defined(MBEDTLS_ECP_NO_INTERNAL_RNG))
+#error "MBEDTLS_ECP_C requires a DRBG module unless MBEDTLS_ECP_NO_INTERNAL_RNG is defined or an alternative implementation is used"
+#endif
+
 #if defined(MBEDTLS_PK_PARSE_C) && !defined(MBEDTLS_ASN1_PARSE_C)
 #error "MBEDTLS_PK_PARSE_C defined, but not all prerequesites"
 #endif