release.sh: allow GITROOT, etc, to be overridden

[ngie]

Summary

release(7) claims that the various variables, e.g., GITROOT, etc, can
be overridden in config files, but unfortunately that's not true (the
values remained whatever default they were hardcoded to in the script).

This change allows the following variables to be overridden in config
files, permitting the release process to be customizable.

Test Plan

This change allowed me to do the following in a config file:

SRCCONF="$(dirname "$0")/src.conf"
GITROOT="file:///home/ngie/git/"
GITSRC="freebsd-src"
GITPORTS="freebsd-ports"

[cperciva]

Hmm, I thought the way this worked was that env_setup set defaults and then the config file was sourced to override those values.

[ngie]

In D54374#1242935, @cperciva wrote:

Hmm, I thought the way this worked was that env_setup set defaults and then the config file was sourced to override those values.

There's no way to just redefine some of the values in a custom conf file (the conf definition of env_setup would shadow the release.sh provided definition), so instead you end up having to fork the entire function to tweak the behavior for a few variables.

It seems like env_setup should be named default_env_setup and env_setup should be a no-op function that allows others to override the values. I would be game for doing that instead since that would be a lot cleaner/easier to document in release(7).

[cperciva]

Ok, I'm going to have to refresh my memory of how these bits work.