mbedTLS: Add ESP8266 hardware entropy source discovered by @foogod

Addresses #3, provided turns out to be an effective HWRNG.

extras/mbedtls/hardware_entropy.c

@@ -0,0 +1,27 @@
+/* ESP8266 "Hardware RNG" (validity still being confirmed) support for ESP8266
+ *
+ * Based on research done by @foogod.
+ *
+ * Please don't rely on this too much as an entropy source, quite yet...
+ *
+ * Part of esp-open-rtos
+ * Copyright (C) 2015 Angus Gratton
+ * BSD Licensed as described in the file LICENSE
+ */
+#include <mbedtls/entropy_poll.h>
+#include <esp/wdev_regs.h>
+#include <string.h>
+
+int mbedtls_hardware_poll( void *data,
+                           unsigned char *output, size_t len, size_t *olen )
+{
+    (void)(data);
+    for(int i = 0; i < len; i+=4) {
+        uint32_t random = WDEV.HWRNG;
+        /* using memcpy here in case output is unaligned */
+        memcpy(output + i, &random, (i+4 <= len) ? 4 : (len % 4));
+    }
+    if(olen)
+        *olen = len;
+    return 0;
+}