Build a Zephyr project with an out-of-tree driver

Hi I'm trying to build a project with an out-of-tree driver along with the custom board. I used the example app repo to prepare this directory sturcture:

.
├── app
│   ├── CMakeLists.txt
│   ├── CMakeLists.txt.bkp
│   ├── Kconfig
│   ├── my_custom_board.overlay
│   ├── prj.conf
│   └── src
│       └── main.c
├── boards
│   └── others
│       └── my_custom_board
│           ├── board.cmake
│           ├── board.yml
│           ├── Kconfig
│           ├── Kconfig.my_custom_board
│           ├── my_custom_board_defconfig
│           ├── my_custom_board.dts
│           ├── my_custom_board-pinctrl.dtsi
│           └── my_custom_board.yaml
├── CMakeLists.txt
├── drivers
│   └── sensor
│       ├── CMakeLists.txt
│       ├── Kconfig
│       ├── lsm6dsl.c
│       ├── lsm6dsl.h
│       ├── lsm6dsl_shub.c
│       ├── lsm6dsl_spi.c
│       └── lsm6dsl_trigger.c
├── dts
│   └── bindings
│       ├── st,mylsm6dsl-common.yaml
│       └── st,mylsm6dsl-spi.yaml
├── include
│   └── app
│       └── drivers
│           └── lsm6dsl.h
├── Kconfig
├── west.yml
└── zephyr
    └── module.yml

The contents of Kconfig, CMakeLists.txt, zephyr/module.yml, app/CMakeLists.txt are as per the provided example.

I can see that zephyr/module.yml declares the location of the custom board and bindings:

build:
  # Path to the Kconfig file that will be sourced into Zephyr Kconfig tree under
  # Zephyr > Modules > example-application. Path is relative from root of this
  # repository.
  kconfig: Kconfig
  # Path to the folder that contains the CMakeLists.txt file to be included by
  # Zephyr build system. The `.` is the root of this repository.
  cmake: .
  settings:
    # Additional roots for boards and DTS files. Zephyr will use the
    # `<board_root>/boards` for additional boards. The `.` is the root of this
    # repository.
    board_root: .
    # Zephyr will use the `<dts_root>/dts` for additional dts files and
    # `<dts_root>/dts/bindings` for additional dts binding files. The `.` is
    # the root of this repository.
    dts_root: .

And Kconfig declares the custom driver location:

rsource "driver/sensor/Kconfig"

When I run west build -b my_custom_board app I get Invalid BOARD error. I see that the instructions say to initialise with west update, but I already have the zephyr codebase as below structure and I don't want to download the entire thing.

zephyrproject
 |
 ├── zephyr
 │
 └── application
      │
      └── my_example_application

How can I get the custom board, bindings and the driver recognised? Thank you.