kernel plugin: properly generate Ubuntu kernel .config

Recent Ubuntu kernels (>= Jammy) are now using a new format to store
their .config, called annotations [1].

The kernel plugin is trying to assemble a .config using the old config
chunks stored in `debian.<kernel>/config/*` that are not available
anymore in the kernels that are using this new annotations model.

Instead of trying to manually assemble the config we should rely on
`debian/rules genconfigs` that is more portable across different Ubuntu
kernels and releases.

[1] https://lists.ubuntu.com/archives/kernel-team/2023-June/140230.html

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

snapcraft_legacy/plugins/v2/_kernel_build.py

@@ -278,17 +278,20 @@ def _do_base_config_cmd(
  logger.info("Using ubuntu config flavour %s", config_flavour)
  conf_cmd = textwrap.dedent(
  """ echo "Assembling Ubuntu config..."
- branch=$(cut -d'.' -f 2- < ${{KERNEL_SRC}}/debian/debian.env)
- baseconfigdir=${{KERNEL_SRC}}/debian.${{branch}}/config
- archconfigdir=${{KERNEL_SRC}}/debian.${{branch}}/config/${{DEB_ARCH}}
- ubuntuconfig=${{baseconfigdir}}/config.common.ubuntu
- archconfig=${{archconfigdir}}/config.common.${{DEB_ARCH}}
- flavourconfig=${{archconfigdir}}/config.flavour.{config_flavour}
- cat ${{ubuntuconfig}} ${{archconfig}} ${{flavourconfig}} \
-> {dest_dir}/.config 2>/dev/null || true
  if [ -f ${{KERNEL_SRC}}/debian/rules ] && [ -x ${{KERNEL_SRC}}/debian/rules ]; then
+ # Generate Ubuntu kernel configs
+ pushd ${{KERNEL_SRC}}
+ fakeroot debian/rules clean genconfigs || true
+ popd
+
+ # Pick the right kernel .config for the target arch and flavour
+ ubuntuconfig=${{KERNEL_SRC}}/CONFIGS/${{DEB_ARCH}}-config.flavour.{config_flavour}
+ cat ${{ubuntuconfig}} > ${dest_dir}/.config
+
+ # Clean up kernel source directory
  pushd ${{KERNEL_SRC}}
  fakeroot debian/rules clean
+ rm -rf CONFIGS/
  popd
  fi""".format(
  config_flavour=config_flavour, dest_dir=dest_dir