skeleton: Clean up profile file

There's a lot of code in /etc/profile, which doesn't really belong in a
minimal default skeleton.

Also, add an 'unset i' to avoid clutter.

If the user has a specific needs, it needs to be added in
/etc/profile.d/ by a post-build script.

Signed-off-by Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>