Fix #229: Match Arduino flash layout

[danepowell]

According to the docs this should align the flash layout to the Arduino IDE defaults:

• https://github.com/esp8266/Arduino/blob/master/tools/sdk/ld/eagle.flash.4m2m.ld
• https://arduino-esp8266.readthedocs.io/en/latest/filesystem.html#spiffs-and-littlefs

I tested this and verified that upgrading a Wemos D1 Mini via the web UI works great under both possible conditions:

• When the existing firmware was uploaded using default Arduino IDE settings
• When the existing firmware was uploaded from platform.io via the web UI

[gysmo38]

Thank you :)