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.

examples/http_get/main.c

@@ -7,6 +7,8 @@
 #include "espressif/esp_common.h"
 #include "espressif/sdk_private.h"
 
+#include <string.h>
+
 #include "FreeRTOS.h"
 #include "task.h"
 

examples/http_get_ssl/main.c

@@ -9,6 +9,8 @@
 #include "espressif/esp_common.h"
 #include "espressif/sdk_private.h"
 
+#include <string.h>
+
 #include "FreeRTOS.h"
 #include "task.h"
 

examples/tests/hmac_test_vectors/main.c

@@ -12,6 +12,8 @@
 #include "FreeRTOS.h"
 #include "ssl.h"
 
+#include <string.h>
+
 struct test_vector {
     const uint8_t *key;
     const uint8_t key_len;