esp8266/scripts/flashbdev: Use all available Flash for filesystem.

All Flash sans firmware at the beginning and 16K SDK param block at the
end is used for filesystem (and that's calculated depending on the Flash
size).

esp8266/scripts/flashbdev.py

@@ -60,7 +60,9 @@ if 22 >= size >= 18:
         machine.reset()
         while 1: time.sleep(1)
 
-if esp.flash_size() < 1024*1024:
+size = esp.flash_size()
+if size < 1024*1024:
     bdev = None
 else:
-    bdev = FlashBdev()
+    # 16K at the flash end is reserved for SDK params storage
+    bdev = FlashBdev((size - 16384) // FlashBdev.SEC_SIZE - FlashBdev.START_SEC)