Better manage Heads kernel config requirements

[JonathonHall-Purism]

Heads requires a fair amount of specific kernel configuration. However, that is all duplicated among the various Linux config files, many of which are board-specific. (Librem devices share one, except L1UM v1 which has its own.)

We should move in a direction of establishing the kernel config required by Heads separate, separate from the drivers added by each board.

Maybe a place to start could be:

• have a "base config" for each kernel version currently in use
• have "board configs" specify the additional configs to enable, maybe apply this during with Linux's scripts/config (this would be comparable to the existing Linux configs, many of which are board-specific but some of which are shared between some related boards)

The intent would be that "board configs" enable hardware-related drivers, but in an initial phase it could just capture whatever differences we currently have.

That'd reduce the duplication of Heads' required configs to just the number of distinct kernel versions in use, which is far fewer than the number of Linux configs we have.

Related to #1626 (comment)