Document cross-compile / USER_LOCAL_PREFIX better

Summary

Explain how this and CMAKE_INSTALL_PREFIX act when doing cross-compile or user-install builds, together with USER_LOCAL_INCDIRS and USER_LOCAL_LIBDIRS.

Handling of udev rules needs to be fixed for this case too.

Issue Planning (for developers only)

StartDate: 2020-04-01

DueDate: 2020-04-01