Fix debconf variable name

Signed-off-by: Marco Marinello <mmarinello@sezf.it>

debian/config

@@ -2,6 +2,6 @@
 
 . /usr/share/debconf/confmodule
 
-db_input high fuss-fucc/tftp_directory || true
+db_input critical fuss-fucc/sftp_directory 
 db_go
 

debian/templates

-Template: fuss-fucc/tftp_directory
+Template: fuss-fucc/sftp_directory
 Type: string
 Default: /srv/clonezilla
 Description: Directory mounted via SSH by clonezilla