Add comment for Linux based OSs

Rather than assume the location of the user's sketchbook folder (which may not be correct and is different depending on OS), just tell them how to find the location if they don't already know it. This makes the instructions more simple and less confusing.

When present, use a tools-installed python executable for any python
calls.

When tools/upload.py is present, use it in place of esptool-ck.exe.

When neither of these are present, as in releases 2.5.0 and prior, use
the prior behavior, unchanged.

Instead of a bunch of nested if-elses, use a list of strings to try
iterating over to fins python(.exe).

Replace 1-line if-else with ternary operator

The core moved to python3 because python (2) is EOL on January 1, 2020.
Move the search paths and platform variables to use python3 instead.

Remove the no longer needed --end parameter to upload.py

Fix uploader for python3 compat and core 2.6.x

Update README for Linux based OSs