This tftpboot.img on SPARC, I assume the installer is the
same across architectures.

When installing over an FTP server, the installer connects
first (to download base/hdlist and base/comp). Then it
presents the list of packages, waits for the user to check
which packages he/she wants -- then assumes the connection
to the FTP server is still open. In reality, package
selection may take a bit longer than 300 seconds (or even
900 seconds). At this point, the installer is unable to
get even the first package, dies, and shuts down.

Verified: just accepting the default package selection,
the installation gets underway with no problems.

Fix: disconnect before, and reconnect after, package
selection to the ftp server