doc: Clean up build instructions and use of variables

Remove most unnecessary instances of `export` and `cmake` use that can
instead be replaced with `zephyr-app-commands` or similar. This is to
avoid documentation using different mechanisms to describe the same
actions and in preparation for documenting `west build` everywhere.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
5 files changed