Fix sgx porting issues: incorrect compile flags, porting func impl, document, etc. (#145)

* Fix sgx porting issues: incorrect compilation flags, porting function impl, document, etc.

* Update bh_platform.c

Add check for function bh_vprintf_sgx: check whether print_function is not NULL.

core/shared-lib/utils/runtime_timer.c

@@ -233,7 +233,7 @@ timer_ctx_t create_timer_ctx(timer_callback_f timer_handler,
         release_timer_list(&ctx->free_timers);
         bh_free(ctx);
     }
-    printf("timer ctx create failed\n");
+    PRINT("timer ctx create failed\n");
     return NULL;
 }