- .config is now compared with previous one byte-wise, diff shown, and mtime preserved directly via `cp -p`
- this does away with the previous mtime-based BS I had written during the fasthash era (and thus remove file-mtime.sh, regen lib)
- split some functions, to better control the interactive piece in the _middle_ of kernel config process
- no more `kernel_config_maybe_interactive()`, now `kernel_config()` that controls its own logging sections
- extract `kernel_determine_toolchain()` out of config into make
- introduce internal hook `armbian_kernel_config()` that runs before custom version (for .config defaults, coming soon)
