Allowing flashing mode to be selected by the user

[abishekmuthian]

Greetings developers & contributors.

LoLin NodeMCU V3 is a widely available, Cheapest ESP8266 board and uses 26MHz crystal(Even on latest boards) and so the flashing mode needs to be set to 0x0040. As it's being done in the esptool.

Without this, flashing using esp-web-tools results in reset loop as the header patching was removed in esp-web-flasher https://github.com/NabuCasa/esp-web-flasher/pull/86 as it was perceived to be useless for ESP32 boards.

The release note for the change states,

It's up to the caller of the library to provide the right values

So in this case esp-web-tools is the caller and I think the best way to address this is for allowing the user to select different flashing mode via a setting in the manifest.