Traditionally per-user configuration files have been placed
in user's home directory with their names beginnig with a dot
to hide them from some tools like ls(1). However, the number
of programs following this convention have grown over time
to the point where the number of hidden files becomes inconvenient to
some users. For this reason the XDG Base Directory Specification[1]
specifies an alternate place to store configuration files under
~/.config directory.

This patch enables avrdude to look for ~/.config/avrdude/config
configuration file, if ~/.avrduderc doesn't exist.

[1] https://specifications.freedesktop.org/basedir-spec/basedir-spec-0.8.html

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