Replace all binary SDK libc functions with newlib

Adds a new build step to remove the SDK object files listed in
<libname>.remove.

Closes #1.

FreeRTOS/Source/portable/esp8266/sdk_compat.c

@@ -1,14 +0,0 @@
-/*
- * Wrappers for functions called by binary espressif libraries
- *
- * Part of esp-open-rtos
- * Copyright (C) 2015 Superhouse Automation Pty Ltd
- * BSD Licensed as described in the file LICENSE
- */
-#include <stdlib.h>
-#include <string.h>
-#include "FreeRTOS.h"
-
-void *zalloc(size_t nbytes) {
-    return calloc(1, nbytes);
-}