Prevent stand-alone tftpd on Debian

The tftpd-hpa package includes a SysV startup script. Ensure
(rather brutally) that this is turned off, to let xinetd get a
look-in.

Change-Id: I01f34c6869b60432cff115e46246cd526ec12a62
Merge "Prevent stand-alone tftpd on Debian"