Upgrade base OpenBSD 5.4 to 5.5 amd64 with time_t change without using bsd.rd (headless)

cd /home
ftp ftp://ftp.eu.openbsd.org/pub/OpenBSD/snapshots/amd64/bsd
ftp ftp://ftp.eu.openbsd.org/pub/OpenBSD/snapshots/amd64/base54.tgz
echo '/usr/sbin/pwd_mkdb -d /etc /etc/master.passwd' >> /etc/rc.firsttime
cp `which reboot` /root/
cd /
cp /home/bsd /
tar xzpmf /home/base54.tgz
/root/reboot

don't forget to 'clean' lastlog, wtmp and spamdb:
cat /dev/null > /var/log/wtmp
cat /dev/null > /var/log/lastlog
cat /dev/null > /var/db/spamd