Fix "method_complexity" issue in src/elivepatch_server/resources/livepatch.py

[aliceinwire]

Function build_kernel has a Cognitive Complexity of 7 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/gentoo/elivepatch-server/src/elivepatch_server/resources/livepatch.py#issue_5d277a45d24643000100001d