system/Config.in: add new init - openrc

system/Config.in: add new init - openrc

This is very basic settings for openrc init.

* system/Config.in
  Allows to select openrc as init system (which auto selects
  openrc-skeleton and openrc package).

* package/ifupdown-scripts/Config.in
  openrc has its own service to bring up/down interfaces, so
  ifupdown-scripts should not be enabled when openrc is enabled to
  prevent service clash.

Signed-off-by: Michał Łyszczek <michal.lyszczek@bofc.pl>
[Thomas: take into account the !BR2_STATIC_LIBS dependency]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>