Fixes for the initial wifi/sensor configuration upload

[Phaze-III]

This PR has some small fixes to make the Configuration part of the airrohr-flasher work as advertised.

• Create a 3M spiffs partition instead of 1M. The firmware expects a 3M partition and will not recognize a 1M partition created after a full erase. Also OTA updates require 1.5M free space.

• Actually generate a default fs_ssid as airRohr-\<chipid>. An empty fs_ssid in config.json results in firmware Exceptions because the fs_ssid config setting is also used for the hostname/MDNS-name and can't be empty. This fixes https://github.com/opendata-stuttgart/sensors-software/issues/983

[ricki-z]

@Phaze-III changing the SPIFFS layout to 3MB firmware and 1MB data would disable most of the devices actually installed. These devices are formatted 1MB/3MB. This works as we use a 2 step OTA process with a much smaller intermediate firmware that will load the final firmware update.

[Phaze-III]

@ricki-z The 2 step OTA process downloads the new and intermediate firmware to the SPIFFS partition so that one has to be 3M. Basically the PR creates a 1M/3M partioning as needed instead of 1M/1M.

[ricki-z]

@Phaze-III Sorry, my fault. The 1M/3M layout was the default earlier. This seems to have changed in the ESP8266 library. So thanks for the pull request.