Refactor Hardware RNG functions to esp/hwrand.h

As suggested by @foogod, thanks!

core/include/esp/hwrand.h

@@ -0,0 +1,22 @@
+/** esp/hwrand.h
+ *
+ * Hardware Random Number Generator functions.
+ *
+ * For documentation, see http://esp8266-re.foogod.com/wiki/Random_Number_Generator
+ *
+ * Part of esp-open-rtos
+ * Copyright (C) 2015 Angus Gratton
+ * BSD Licensed as described in the file LICENSE
+ */
+#ifndef _ESP_RNG_H
+#define _ESP_RNG_H
+#include <stdint.h>
+#include <sys/types.h>
+
+/* Return a random 32-bit number */
+uint32_t hwrand(void);
+
+/* Fill a variable size buffer with data from the Hardware RNG */
+void hwrand_fill(uint8_t *buf, size_t len);
+
+#endif