Drop support for i586

Drop support for i586

This patch removes support for i586 according to the decision being
taken over a year ago.

It removes the architecture from the build system and removes all
required hacks and other quirks that have been necessary before.

There is no need to ship any changed files to the remaining
architectures as the removed code branches have not been used.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>