Simplify OTA API

src/flash.h

@@ -0,0 +1,13 @@
+#include "arch.h"
+
+struct mg_flash {
+  void *start;   // Address at which flash starts
+  size_t size;   // Flash size
+  size_t align;  // Write alignment
+  bool (*write_fn)(void *, const void *, size_t);  // Write function
+  bool (*swap_fn)(void);                           // Swap partitions
+};
+
+bool mg_ota_flash_begin(size_t new_firmware_size, struct mg_flash *flash);
+bool mg_ota_flash_write(const void *buf, size_t len, struct mg_flash *flash);
+bool mg_ota_flash_end(struct mg_flash *flash);