embedded-linux: improve VS code lab

[tpetazzoni]

There are a couple of things that are not ideal:

• The .vscode files contain many hardcoded paths that have to be tweaked, the IP address of the target is duplicated in prep-debug.sh and the .vscode files
• The .vscode files are delivered "already done" which goes against the principle of our training courses that we explain how things are done. These files seem to be a bit magical and not explained
• The actual segfault of our application is not very well investigated by the means of the VS Code debugger. We tell participants "look at the segfault" and then "look at the code, the bug will be obvious"... making the debugger somewhat useless in that particular situation.