Working TFTP server based OTA updates

Tested with 16MBit flash configuration, two rboot update slots.

Closes #10

core/include/esp/rom.h

@@ -0,0 +1,21 @@
+/* "Boot ROM" function signatures
+
+   Note that a lot of the ROM functions used in the IoT SDK aren't
+   referenced from the Espressif RTOS SDK, and are probably incompatible.
+ */
+#ifndef _ESP_ROM_H
+#define _ESP_ROM_H
+
+#include <stdint.h>
+
+void Cache_Read_Disable(void);
+
+/* http://esp8266-re.foogod.com/wiki/Cache_Read_Enable
+
+   Note: when compiling non-OTA we use the ROM version of this
+   function, but for OTA we use the version in extras/rboot-ota that
+   maps the correct flash page for OTA support.
+ */
+void Cache_Read_Enable(uint32_t odd_even, uint32_t mb_count, uint32_t no_idea);
+
+#endif