package/ifupdown-scripts: add openrc support

package/ifupdown-scripts: add openrc support

The "network" script allows configuring an interface by means of two
/etc/ifup.<if_name> and /etc/ifdown.<if_name> files with the commands
required to bring it up and down, respectively. So we can support the
BR2_SYSTEM_DHCP config if netifrc is not selected.

- Replace the dependency on !BR2_PACKAGE_OPENRC in Config.in by a
  dependency on !BR2_PACKAGE_NETIFRC.

- Add an IFUPDOWN_SCRIPTS_DHCP_OPENRC macro to create the ifup.<if_name>
  and ifdown.<if_name> files.

- Add an IFUPDOWN_SCRIPTS_INSTALL_INIT_OPENRC hook that includes the
  IFUPDOWN_SCRIPTS_{PREAMBLE,DHCP,DHCP_OPENRC} macros. LOCALHOST is not
  required, since "lo" is configured by the "loopback" script.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
[yann.morin.1998@free.fr:
  - adapt after the preamble was spit to itw own patch
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>