tftpd-hpa: fix premature exit

standalone daemon exits as recipe installs file as tftpd-hpa

* Changed script so it can consult /etc/default, as daemon name may be
 subject to change.
* Changed the default name to match the recipe

Signed-off-by: David Turgeon <david.turgeon@gmail.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>