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.

core/sdk_compat.c

@@ -0,0 +1,28 @@
+/*
+ * 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 <stdio.h>
+
+void *zalloc(size_t nbytes)
+{
+    return calloc(1, nbytes);
+}
+
+/* this is currently just a stub to see where in the SDK it gets
+   called, and with what arguments...
+
+   In the binary SDK printf & ets_printf are aliased together, most
+   references appear to be to printf but libphy, libwpa & libnet80211
+   all call ets_printf sometimes... Seems to not be in common code
+   paths, haven't investigated exactly where.
+*/
+int ets_printf(const char *format, ...)  {
+    return printf("ets_printf format=%s\r\n", format);
+}
+