boards: Replace use of `CFLAGS` with configs

[crawfxrd]

Add configs for boards to replace the direct manipulation of CFLAGS.

For configs I have added, I have been using the prefix CONFIG_ to match Kconfig.

The idea is to eventually have a proper tool for managing build configs, such as using Kconfig itself.

[crawfxrd]

coreboot uses Kconfig imported from Linux with patches: util/kconfig

Zephyr appears to have a Python based implementation for parsing Kconfig files: scripts/kconfig

There are Rust bindings for Kconfig via tree-sitter: tree-sitter-kconfig