boards: board revision 0 (when default) does not include revision file

[thedjnK]

Describe the bug
The documentation on https://docs.zephyrproject.org/latest/hardware/porting/board_porting.html#multiple-board-revisions states the following:

Currently, <revision> can be either a numeric MAJOR.MINOR.PATCH style revision like 1.5.0, an integer number like 1, or single letter like A, B, etc. Zephyr provides a CMake board extension function, board_check_revision(), to make it easy to match either style from revision.cmake.

Consider the example of having a board with 2 board revisions, let's call it plank and the two revisions being 0 and 1, then create 2 overlay files in the board directory plank_0.overlay and plank_1.overlay, with the following revision.cmake file:

board_check_revision(FORMAT NUMBER
                     DEFAULT_REVISION 0
                     HIGHEST_REVISION 1
                     VALID_REVISIONS 0 1
)

When configuring for board plank@1, the plank_1.overlay file is correctly included, however when configuring for plank, the build system correct output that it is using revision 0, but the plank_0.overlay file is completely ignored, this is best illistrated by just having a file full of garbage, one would expect cmake to error out, but no error occurs (this is using sysbuild):

-- Zephyr version: 3.5.99 (/tmp/net/zephyr)
-- Found west (found suitable version "1.1.0", minimum required is "0.14.0")
-- Board: plank, Revision: 0
-- ZEPHYR_TOOLCHAIN_VARIANT not set, trying to locate Zephyr SDK
-- Found host-tools: zephyr 0.16.3 (/opt/zephyr-sdk-0.16.3)
...
-- Including signing script: /tmp/net/zephyr/cmake/mcuboot.cmake
-- Configuring done (4.4s)
-- Generating done (0.1s)
-- Build files have been written to: /tmp/net/net_loader/app/_AA/app
--

To Reproduce
Create a board with revision 0 (and another revision), create a revision 0 overlay, try configuring project, overlay is unused.

Expected behavior
Overlay to be used

Impact
Annoyance

Environment (please complete the following information):

• OS: Arch linux x64
• Commit SHA or Version used: 1dc6279