tftpd-hpa: update default tftpboot location

The tftp-hpa package installs /var/lib/tftpboot and tftpd-hpa is
configured to use that directory, but the default points at /svc/tftpboot.
Correct that so the default points to a location that exists when the
package is installed.

Signed-off-by: Joe MacDonald <joe@deserted.net>