cmake: modules: boards: Enhance board aliases for HWMv2

To address concerns about lengthy board identifiers in HWMv2, the
proposal is to apply the existing BOARD_ALIAS feature, like so:

   set(<alias>_BOARD_ALIAS <board>/<soc>)

It should then be possible to build with either:

   -DBOARD=<alias>            # expands to <board>/<soc>
   -DBOARD=<alias>/<variant>  # expands to <board>/<soc>/<variant>

However, this wouldn't work out of the box. A board alias can only be
expanded to a board name, without revision or identifier, because the
alias substitution happens after having parsed BOARD as user input -
namely, into BOARD (name), BOARD_REVISION, and BOARD_IDENTIFIER.

Furthermore, this means that in the legacy model, it was possible to
build with `-DBOARD=<alias>@<revision>`, and it would resolve to the
actual board name + revision.

To support both the old and new use cases, we can parse the alias just
like BOARD itself, then concatenate their identifiers as shown above.
Adding a revision works as before, but now it is also possible for the
alias to set its own revision. In this example:

   set(<alias>_BOARD_ALIAS <board>@<rev-A>/<soc>/<variant>)

`<rev-A>` is treated as the default revision, and it can be overridden:

   -DBOARD=<alias>          # expands to <board>@<rev-A>/<soc>/<variant>
   -DBOARD=<alias>@<rev-B>  # expands to <board>@<rev-B>/<soc>/<variant>

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>