dropbear: dropbear.service: /etc/default/dropbear is optional

The environment file is for optional customization, so don't error out if it
isn't present.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dropbear/dropbear.service

@@ -19,7 +19,7 @@ if [ -L /etc/dropbear \
         mkdir -p "$(readlink /etc/dropbear)"; \
     fi; \
 fi'
-EnvironmentFile=/etc/default/dropbear
+EnvironmentFile=-/etc/default/dropbear
 ExecStart=/usr/sbin/dropbear -F -R $DROPBEAR_ARGS
 ExecReload=/bin/kill -HUP $MAINPID