HWMV2: Support having default variants for board/soc

[decsny]

Is your enhancement proposal related to a problem? Please describe.

A problem with the mimxrt1050_evk and mimxrt1060_evk boards is that they have variants once #79517 merges to describe which flash on the board (requiring board rework to change) should be targeted by the build for the soc. This means the base board mimxrt1050_evk/mimxrt1052 does not build as a valid target when given to west build command.

Describe the solution you'd like

What would be nice would be to be able to specify a default variant for the out of the box setup of the board. For example, the mimxrt1050_evk comes set up for hyperflash out of box, and the mimxrt1060_evk is set up for qspi out of box.

Describe alternatives you've considered

An alternative solution to deal with these boards (and maybe an orthogonal enhancement request) is to be able to specify multiple revisions of a board. I.e., something like mimxrt1060_evk@B@qspi ? Because reworking a board is kind of like making a revision, but the issue here is that the board already has revisions A, B, C, etc, and Zephyr HWM does not support "multi dimensional" revisions like that.