devtool: extract: refactor kern-tools-native handling

devtool: extract: refactor kern-tools-native handling

When extracting linux-yocto kernel source, we don't need to dance around
shutting down and starting up tinfoil anymore, we can just execute the
tasks as needed when needed using tinfoil's new build_targets()
function. This allows us to tidy up the code structure a bit.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>