Add static analysis for defconfigs

[rossburton]

It would be desirable for kernelci to do static analysis on the defconfigs, for example if a config option is removed:

torvalds/linux@802b832

but users of it remain:

https://github.com/torvalds/linux/blob/231825b2e1ff6ba799c5eaf396d3ab2354e37c6b/arch/arm64/configs/defconfig#L1355

then that should be raised in CI.

In the Yocto Project we have a tool which does this: https://git.yoctoproject.org/yocto-kernel-tools/tree/tools/symbol_why.py and we'd be happy to help because our kernel builds fail due to issues like this.

[zeddii]

The script does have some extra functionality, and could use a tweak to work with defconfigs (vs fragments), but there's nothing super high tech in the script itself. I'd be happy to support as needed (versus writing something new and custom, if there's interest.