invalid west cmake diagnostics when using board alias

[pabigot]

Zephyr supports board aliases which allow more natural names to be used for boards at the command line.

Sometime in the past couple weeks use of this feature has started to produce verbose gratuitous warnings. E.g.:

rm -rf build && west build -b pca10056

with

set(pca10056_BOARD_ALIAS nrf52840dk_nrf52840)

in a file identified by the ZEPHYR_BOARD_ALIASES environment variable now produces:

tirzah[165]$ rm -rf build && west build -b pca10056
-- west build: generating a build system
Including boilerplate (Zephyr base): /mnt/nordic/zp/zephyr/cmake/app/boilerplate.cmake
-- Application: /mnt/nordic/zp/zephyr/samples/hello_world
-- Zephyr version: 2.5.99 (/mnt/nordic/zp/zephyr)
-- Found Python3: /usr/bin/python3.8 (found suitable exact version "3.8.5") found components: Interpreter 
-- Found west (found suitable version "0.9.0", minimum required is "0.7.1")
CMake Warning at /mnt/nordic/zp/zephyr/cmake/extensions.cmake:2143 (message):
  

     **********************************************************************
     *
     *                    WARNING
     *
     * CMake variable BOARD set to "nrf52840dk_nrf52840" in:
     *     /mnt/nordic/zp/zephyr/cmake/app/boilerplate.cmake
     *
     * This is too late to make changes! The change was ignored.
     *
     * Hint: BOARD must be set before calling find_package(Zephyr ...).
     *
     **********************************************************************

Call Stack (most recent call first):
  /mnt/nordic/zp/zephyr/cmake/app/boilerplate.cmake:9999 (zephyr_variable_set_too_late)
  /mnt/nordic/zp/zephyr/cmake/app/boilerplate.cmake:233 (set)
  /mnt/nordic/zp/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:24 (include)
  /mnt/nordic/zp/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:35 (include_boilerplate)
  CMakeLists.txt:5 (find_package)


-- Aliased BOARD=pca10056 changed to nrf52840dk_nrf52840
-- Board: pca10056
-- Cache files will be written to: /home/pab/.cache/zephyr
...

The warning is clearly wrong, as the board is being set correctly, and is inappropriate, as the alias was correct.

[tejlmand]

Fixed: #33436