Consistent reference syntax in nextboot v1 action templates

[stephen-soltesz]

Today, epoxy_client action templates can reference: kargs, files, env, and arbitrary vars.

However, the syntax for referencing kargs is different than the others. For example:

 {{kargs `epoxy.ipv4`}}

vs

{{.files.vmlinuz.name}}

The kargs syntax is needed to support arbitrary kernel parameters as keys. Some of which include . or - neither of which can be used in the dot-notation.

Ideally, this would be expressed as a verb (e.g. get_karg, getKarg) or all accesses would use similar syntax (e.g. get_karg, get_filename, get_var, etc.