Previously this script could produce rootfs tarballs suitable for a
few different systems. This worked but meant that several different
scripts were then building root filesystems instead of just using ones
produced by this script. This commit cleans up the script to produce
just a root filesystem. Note that this system is not bootable, that
still needs to be done by another script which processes platform
specific operations. This script just produces a root filesystem for
every architecture that XBPS understands.
