kernel: migrate build rules from Makefile to Kbuild

[dabao1955]

Move all build definitions including object files, compiler flags, version detection logic, and manager signature configuration from kernel/Makefile into a new kernel/Kbuild file.

This aligns with standard Kbuild practices for out-of-tree kernel modules and cleans up the top-level Makefile to only handle high-level targets (all, compdb, clean). The build behavior remains unchanged.

Also retain informational messages for version, manager package, and signature verification to aid debugging and CI.