zephyr-env.sh: Use $XDG_CONFIG_HOME/zephry/zephyrrc if present

According to the [XDG Base Directory Specification] there is a single base
directory in which user configuration files should be stored. This helps
users to keep their home directory uncluttered. This base directory can be
set in the environment variable $XDG_CONFIG_HOME and defaults to
${HOME}/.config if $XDG_CONFIG_HOME is not set.

This commit adjusts the zephyr-env.sh script to look for a user
configuration file in the following locations:

- $XDG_CONFIG_HOME/zephyr/zephyrrc
- $HOME/.config/zephyr/zephyrrc
- $HOME/.zephyrrc

The first one that is found will be used.

[XDG Base Directory Specification]: (https://specifications.freedesktop.org/basedir-spec/latest/

)

Signed-off-by: Andreas Schuster <andreas.schuster@schuam.de>