gadget.yaml: allow optional "dhc-dev-mode" kernel commandline

[mvo5]

This will allow to use the new kernel cmdline options, see: https://github.com/snapcore/snapd/pull/12605/files#diff-febc07409227a4ecf0bc8520f8e7b36fcc2e71677951c6abb1ff78017b53445bR23