boards: add support for string board revisions

[feilongfl]

add support for string board revisions
with chapter a-zA-Z0-9.

The string format now supports revisions for arbitrary strings (including letters, numbers, and dots). However, this format does not support version comparison with HIGHEST_REVISION. Instead, use VALID_REVISIONS for version comparison.

todo:

• add document

[thedjnK]

Before continuing with this, get approval from @tejlmand as I believe support for this was purposely omitted

[tejlmand]

it was mainly omitted because there is no direct need for it at the time, and there is a preference that we should have code which is used within the project, else it's prone to bit-rot.

That said, string as a type is a bit more custom, as two string revisions cannot be directly compared but must use (or imply) EXACT.

But what if someone is actually having a sensible string, like alpha, beta, gamma, delta`, etc.
Then you could actually compare revisions, but not as a greater than string comparison.

This is why we also support a custom revision.cmake implementation, ref: `https://docs.zephyrproject.org/latest/hardware/porting/board_porting.html#custom-revision-cmake-files

This means that testing against a custom list of strings can easily be done as, while at the same time allow users to do more complex testing / adjustments:

set(REVISIONS alpha beta gamma)
if(NOT BOARD_REVISION IN_LIST REVISIONS)
  message(FATAL_ERROR "Invalid revision used, please choose: ${REVISIONS }")
endif()

So strings as revision is supported by Zephyr today, but not supported in the board revision helper function, as the simple case can be just as easily done directly in the revision.cmake file, and more complex cases of matching is harder to do for various types of strings.

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