out-of-installation use of west

User feedback noted that it’s not possible by default to use west commands outside of the installation (i.e. not in a subdirectory of the directory containing .west).

Options:

1. One way to do this is to allow the user to specify the default west installation in a system- or user-level west configuration file. Their locations are described in this comment: https://github.com/zephyrproject-rtos/west/blob/master/src/west/config.py#L27
2. We may also be able to use the WEST_DIR environment variable to cover this case, so build, flash, etc. work normally even if things like the build directory are outside of the installation.
3. Another alternative if WEST_DIR is missing and west is invoked from outside an installation is to fall back on searching inside ZEPHYR_BASE, should that be defined.

Some notes for context:

• you can already build with something like west build -s /tmp/app-source -d /tmp/build and flash with west flash -d /tmp/build even if /tmp/app-source and /tmp/build are outside of the west installation. Just west itself has to be run from inside the installation to find the build and flash commands themselves.
• the use of a west installation, which is found by looking for a .west directory, is a deliberate design decision which allows users to keep parallel installations, cd around between them, and have them ‘just work’, without requiring environment variables which are cumbersome for some, and error-prone in general (since they can point to the wrong place).

[tejlmand]

CMake is already capable of calling out to west if west is present, examples:

    86  if(WEST)
    87    # Verify that the west version found can be used for flashing, e.g. is not the bootstrapper.
    88    execute_process(COMMAND ${WEST} flash -h
    89                    RESULT_VARIABLE return_code
    90                    OUTPUT_QUIET
    91                    ERROR_QUIET

and in PR/13067

 execute_process(
    COMMAND
    west list --format={abspath}\;
    OUTPUT_VARIABLE
    west_project_list
  )

so if west exposes those settings through a call, eg. west config --get <TOOLCHAIN> then CMake can use that info. Environment varibles and command line arguments can take precedence over west config --get, so it stays consistent with what has been discussed earlier and agreed, see https://github.com/zephyrproject-rtos/west/issues/175#issuecomment-459687134 and https://github.com/zephyrproject-rtos/west/issues/175#issuecomment-460299808

Also environment settings can be used if people don’t want to use west

[mbolivar]

• If the option:

  [zephyr]
  zephyr_base = /path/to/zephyr
  

  is set, then use that path

On second thought, I think we need to drop this one. The problem is that the bootstrapper needs to find west, and it doesn’t have access to the config module.

Once we have a unified multi-repo tool in what gets shipped via pypi, rather than just the bootstrapper, we can add that. For now I want to just support ZEPHYR_BASE as a fallback. That should be enough to support backwards compatibility with existing use cases.