Update preparing firmware instructions

[balloob]

@balloob @TD-er Changes ahead, latest Arduino ESP32 stage does not use anymore *.bin file for bootloader. The *.elf file builded with Arduino Lib Builder is used and converted to an *.bin file during compile of the main sketch. So no more wrong SHA message during boot of the ESP32x. Currently Platformio does not support yet.
See issue. This will be the standard way, with release of core 2.0.6.
So in a not so far future, for building a merged firmware, the creator has to do the *.bin build out of the *.elf bootloader file too.

Originally posted by @Jason2866 in #298 (comment)

[Jason2866]

Needed Platformio build part has been updated and merged today in Arduino esp32 repo. Platformio crew has done it very user friendly, the builded bootloader.bin is in the same folder where the builded bin firmware sketch is.

    Offset | File
 -  0x1000 | /Users/hans/Git/Tasmota/.pio/build/tasmota32/bootloader.bin
 -  0x8000 | /Users/hans/Git/Tasmota/.pio/build/tasmota32/partitions.bin
 -  0xe000 | /Users/hans/.platformio/packages/framework-arduinoespressif32/tools/partitions/boot_app0.bin
 -  0x10000 | /Users/hans/Git/Tasmota/.pio/build//tasmota32/firmware.bin

So for the user just the place where to look for the bootloader.bin has changed.

The generated bootloader.bin is only useable for the definitions choosen in used boards.json for flash freq. flash type and mode. Nothing has changed here!
For different parameters a "fitting" bootloader.bin is needed.

[balloob]

Since we didn't document where the user could find bootloader.bin, I guess this doesn't require any changes for our README?

[Jason2866]

Yep. Sometimes it is better not to document everything ;-)

[TD-er]

Yep. Sometimes it is better not to document everything ;-)

That's my specialty :)