Fix OTA upgrade bug and repair saving WiFi credentials

[king2]

1. removed skipping one byte from firmware sent by web
2. size of flash chip now not hardcoded, will be restored correctly, so system parameters will be saved for all flash sizes, not only for 512K

[con-f-use]

Okay people, keep me up to date regarding whether this works or not. See #60, #61 and #62.