rootfs-386.tar.gz: Add with BusyBox 1.25.1

This was extracted with from stage3-i486-20170919.tar.bz2.  The stage3
had a SHA512 hash of [1] and a valid signature by [2].

Generated with:

  $ cd contrib/rootfs-builder
  $ make ARCHES=i486
  $ cp -f rootfs-i486.tar.gz ../../rootfs-386.tar.gz

[1]: 9e007ef33f5aef6f70b7e263ed66e7679479261758ac34f61e5a2558de7559eb9a7fdbb31264bbeba09bfcc06df57263592d986393051bac5cd432c1228b65f0
[2]: Gentoo Linux Release Engineering (Automated Weekly Release Key) <releng@gentoo.org>
     Primary key fingerprint: 13EB BDBE DE7A 1277 5DFD  B1BA BB57 2E0E 2D18 2910

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking