Update cross-compile documentation

[knight-of-ni]

First, I understand that someone prior to me spent a lot of time writing the existing documentation. I do not wish to upset anyone by re-writing the documentation someone else worked hard on.

I am hopeful this PR represents a positive improvement. Let me know if you agree with it.

This PR updates the cross-compile documentation to take a more comprehensive approach. It documents, in as detailed a manner as possible, all the steps required from the initial cloning of the leosac repo to the final step of deploying the cross-compiled files onto the target machine.

[xaqq]

The documentation of leosac is nowhere near perfect, so improvement and even rewrite are welcome.

Note the line sed -i 's/\/usr\/bin\/leosac/\/usr\/local\/bin\/leosac/g' pkg/deb/leosacd is not parsed correctly by doxygen.

Also you reminded me of this old leosacd script. we should make a systemd one.

[knight-of-ni]

Thanks for the heads up about the sed line not rendering. Perhaps it would be better to avoid the whole sed line altogether and instead use a make macro. Let cmake populate the service file with the correct path to the leosac binary during the build.

Note I'm already using a leosac systemd service file I created based on the one found in the arch subfolder. I'll include that as well.