Fix build failure with MBEDTLS_PLATFORM_NV_SEED_ALT

An earlier botched backport (d56ca658ab8bd49cbbde8790240ec4074c0a7622)
had the wrong name for a variable and a missing header inclusion.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_entropy.function b/tests/suites/test_suite_entropy.function
index 3b87244..cb5e204 100644
--- a/tests/suites/test_suite_entropy.function
+++ b/tests/suites/test_suite_entropy.function
@@ -1,6 +1,7 @@
 /* BEGIN_HEADER */
 #include "mbedtls/entropy.h"
 #include "mbedtls/entropy_poll.h"
+#include "mbedtls/md.h"
 
 /*
  * Number of calls made to entropy_dummy_source()
@@ -346,7 +347,7 @@
 
     // Set the initial NV seed to read.
     // Get exactly MBEDTLS_ENTROPY_BLOCK_SIZE bytes from read_str.
-    TEST_ASSERT( strlen( read_seed ) / 2 >= MBEDTLS_ENTROPY_BLOCK_SIZE );
+    TEST_ASSERT( strlen( read_seed_str ) / 2 >= MBEDTLS_ENTROPY_BLOCK_SIZE );
     read_seed_str[MBEDTLS_ENTROPY_BLOCK_SIZE * 2] = '\0';
     unhexify( read_seed, read_seed_str );
     memcpy( buffer_seed, read_seed, MBEDTLS_ENTROPY_BLOCK_SIZE );