Add support for generating kconfig from fragments
This is the latest attempt to implement support for kconfig fragments in pmOS/pmaports, and takes a somewhat different approach than previous attempts [1] [2]
To understand how this is supposed to work, I recommend reading the last 2 commit messages in this branch. This is 'backwards compatible' with how kconfig is currently managed for 99% of kernels in pmaports, it's not a breaking change for anything. I tried to re-use as much as possible from pmbootstrap / pmaports.
Here's an example of how a kernel pkg can use this: pmaports!6648 (merged) . The full config in that MR was generated with this pmb patch, and I would then make changes to the fragment(s) in that kernel's pmaports dir and run pmb kconfig generate any time those change or changes are added to kconfigcheck.toml.