Modularize build system with Kconfig integration

[jserv]

This splits monolithic Makefile into mk/ fragments (toolchain, features, deps, tests, format) and add Kconfiglib-based configuration via 'make config' (interactive menuconfig) or 'make defconfig'.

Features (SLIRP networking, web observatory) are enabled by default and toggled via CONFIG_HAS_WEB in .config. The build mode (debug/release) is also configurable through Kconfig.

Kconfiglib is fetched on demand from sysprog21/Kconfiglib via shallow clone into tools/kconfig/, cleaned by 'make distclean'.

Change-Id: I719856353c77c9c69872b9180fa6d6296b732a6c

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Summary by cubic

Modularized the build into mk/ fragments and integrated Kconfiglib so features and build mode are configured via make config/make defconfig. Builds now require a .config; run make defconfig && make or use the interactive make config.

• New Features

  • make config (menuconfig), make defconfig, make oldconfig, and make savedefconfig generate and manage .config (defaults: CONFIG_HAS_SLIRP=y, CONFIG_HAS_WEB=y, debug build).
  • Feature flags: CONFIG_HAS_SLIRP, CONFIG_HAS_WEB; build mode: CONFIG_BUILD_DEBUG or CONFIG_BUILD_RELEASE; optional CONFIG_LKL_DIR.
  • On-demand Kconfiglib download, pinned into tools/kconfig/; removed by make distclean. LKL and minislirp still auto-fetched when enabled.
  • .config changes trigger object rebuilds.

• Migration

  • Run make defconfig once, then make. Use two steps (e.g., make defconfig && make), not make defconfig all.
  • CI updated to run make defconfig before building/tests.
  • Use make oldconfig when new options are added; make distclean resets .config and removes tools/kconfig/.

^{Written for commit cf507ae. Summary will update on new commits.}