Drop the tftpd binary, which is no longer provided upstream.  Users
should switch to the atftp server as a replacement.

Avoid executing runtime tests, which are not supported in cross-build
environments.

Signed-off-by: Noah Meyerhans <frodo@morgul.net>

The iputils build system embeds git tags into the generated binaries
for use by commands like ping -V.  Since openwrt packaging is done in
a different repository from the upstream repo, the tags it finds
aren't particularly meaningful, and we get confusing results like
those described at https://github.com/openwrt/packages/issues/13920

This change removes the git tag inspection in favor of the static
version string that's already known to the upstream build system.

Signed-off-by: Noah Meyerhans <frodo@morgul.net>