Rename internal MBEDTLS_ENTROPY_HAVE_STRONG to ENTROPY_HAVE_STRONG

This commit renames the test-only flag MBEDTLS_ENTROPY_HAVE_STRONG to ENTROPY_HAVE_STRONG to make it more transparent
that it's an internal flag, and also to content the testscript tests/scripts/check-names.pl which previously complained
about the macro occurring in a comment in `entropy.c` without being defined in a library file.
diff --git a/library/entropy.c b/library/entropy.c
index 10449b8..7c09156 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -83,7 +83,7 @@
     mbedtls_havege_init( &ctx->havege_data );
 #endif
 
-    /* Reminder: Update MBEDTLS_ENTROPY_HAVE_STRONG in the test files
+    /* Reminder: Update ENTROPY_HAVE_STRONG in the test files
      *           when adding more strong entropy sources here. */
 
 #if defined(MBEDTLS_TEST_NULL_ENTROPY)