Missing CI coverage after removal of legacy targets.

[hydra]

Is your feature request related to a problem? Please describe

As highlighted by #12298 there is missing CI coverage for some code gating paths.

Prior to the removal of legacy targets in #11884 there was CI compiler coverage for more code gating combinations than exists today.

Describe the solution you'd like

Possible solutions:

1. compile all unified targets once a day, report compile failures and handle them (no PR integration)
2. ensure PR's are not merged until all unified targets are built by the CI.
3. have the CI run code coverage over all the base targets and combine the results and have it highlight any code that is not compiled, and add CI specific targets that use specific combinations of code gating, or use scripts to accomplish this, for example like the ones here support\scripts\build_*.

Describe alternatives you've considered

See above.

Other information

Note that:

• It's not appropriate to use all code gatings in base targets, such as USE_EXST, USE_QUADSPI.
• It's not possible to use all code gatings at the same time, such as CONFIG_IN_SD_CARD and CONFIG_IN_RAM. The latter is used when bringing up new MCUs before a persistent config storage subsystem is available (such as flash, sd or external flash).

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