Firmware upgrade process documented

[kasparsd]

I've been able to capture the HTTP traffic to the unit during a firmware update which I have documented here cmroche/greeclimate#111 and in a blog post.

In essence, the app sends the following pack payload encrypted with the device key (not the hardcoded global key):

{"t":"upgrade","url":"http://eu.grih.gree.com/wifiModule/image/10141/145680","mac":"UNIT_MACID"}

It is important to note that the firmware URLs include a FileMd5 HTTP header which appears to be used for validating the integrity of the downloaded file (based on the strings extracted from the binaries). Therefore, using a custom update server would require adding this header.

My unit currently fails to apply the latest official 3.77 update so I'm unable to verify this upgrade process. However, I was able to send a custom update payload to the unit and get the confirmation message {"t":"ret","r":200,"mac":"UNIT_MACID"} back.