CTR_DRBG: support set_entropy_len() before seed()

mbedtls_ctr_drbg_seed() always set the entropy length to the default,
so a call to mbedtls_ctr_drbg_set_entropy_len() before seed() had no
effect. Change this to the more intuitive behavior that
set_entropy_len() sets the entropy length and seed() respects that and
only uses the default entropy length if there was no call to
set_entropy_len().

The former test-only function mbedtls_ctr_drbg_seed_entropy_len() is
no longer used, but keep it for strict ABI compatibility.
diff --git a/tests/suites/test_suite_ctr_drbg.function b/tests/suites/test_suite_ctr_drbg.function
index 4a97826..01050d9 100644
--- a/tests/suites/test_suite_ctr_drbg.function
+++ b/tests/suites/test_suite_ctr_drbg.function
@@ -44,11 +44,11 @@
 
     /* CTR_DRBG_Instantiate(entropy[:entropy->len], nonce, perso, <ignored>)
      * where nonce||perso = nonce[nonce->len] */
-    TEST_ASSERT( mbedtls_ctr_drbg_seed_entropy_len(
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, entropy_chunk_len );
+    TEST_ASSERT( mbedtls_ctr_drbg_seed(
                      &ctx,
                      mbedtls_test_entropy_func, entropy->x,
-                     nonce->x, nonce->len,
-                     entropy_chunk_len ) == 0 );
+                     nonce->x, nonce->len ) == 0 );
     if( reseed_mode == RESEED_ALWAYS )
         mbedtls_ctr_drbg_set_prediction_resistance(
             &ctx,