atftp: correctly handle init script

else it's affected to atftp package which is the client
and not the server, but the init file is only in the server
package thus leading to the following log at boot :
Configuring atftp.
update-rc.d: /etc/init.d/atftpd: file does not exist

while at it, there is a cosmetic change from _atftpd
to _${PN}d to make the recipe consistent

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>