doc: west: add FAQ for missing extension command

This page covers common issues with west and how to solve them.

"invalid choice: 'build'" (or 'flash', etc.)

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If you see an unexpected error like this when trying to run a Zephyr extension

command (like :ref:`west flash <west-flashing>`, :ref:`west build

<west-building>`, etc.):

.. code-block:: none

   $ west build [...]

   west: error: argument <command>: invalid choice: 'build' (choose from 'init', [...])

   $ west flash [...]

   west: error: argument <command>: invalid choice: 'flash' (choose from 'init', [...])

The most likely cause is that you're running the command outside of a

:ref:`west workspace <west-workspace>`. West needs to know where your workspace

is to find :ref:`west-extensions`.

To fix this, you have two choices:

#. Run the command from inside a workspace (e.g. the :file:`zephyrproject`

   directory you created when you :ref:`got started <getting_started>`).

   For example, create your build directory inside the workspace, or run ``west

   flash --build-dir YOUR_BUILD_DIR`` from inside the workspace.

#. Set the :envvar:`ZEPHYR_BASE` :ref:`environment variable <env_vars>` and re-run

   the west extension command. If set, west will use :envvar:`ZEPHYR_BASE` to

   find your workspace.

If you're unsure whether a command is built-in or an extension, run ``west

help`` from inside your workspace. The output prints extension commands

separately, and looks like this for mainline Zephyr:

.. code-block:: none

   $ west help

   built-in commands for managing git repositories:

     init:                 create a west workspace

     [...]

   other built-in commands:

     help:                 get help for west or a command

     [...]

   extension commands from project manifest (path: zephyr):

     build:                compile a Zephyr application

     flash:                flash and run a binary on a board

     [...]

"invalid choice: 'post-init'"

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