dropbear: fix procd interface trigger install

Install procd interface triggers only for interfaces which are enabled
so dropbear instances running on (an) enabled interface(s) are not
restarted due to an interface trigger of an interface which is disabled.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
(cherry picked from commit e5bbead1a8)

package/network/services/dropbear/files/dropbear.init

@@ -109,7 +109,9 @@ keygen()
 load_interfaces()
 {
 	config_get interface "$1" Interface
-	interfaces=" ${interface} ${interfaces}"
+	config_get enable "$1" enable 1
+
+	[ "${enable}" = "1" ] && interfaces=" ${interface} ${interfaces}"
 }
 
 start_service()