Ensure all ifconfig file resources will be created before any L3_if_downup

W/o this patch, the main interface's ifconfig file creation
will *immediately* trigger the L3_if_downup event despite on
other subinterfaces desired startup order.

The patch clearly separates interfaces' configuration from startup,
as well as allows network_setup manifest to control interfaces'
configuration and startup order.

Fixes: #PRD-2091

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>