skeleton: add a default /etc/shells
board/ryco: add postscript.sh
set simple network setup via the system configuration submenu

This patch allows the setup of simple /etc/network/interfaces (or
/etc/systemd/network/80-buildroot.network if networkd is enabled) via the
configuration menus instead of using an overlay

* supports disabling the interface
* supports manual ipv4 configuration
* supports dhcp configuration

Signed-off-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
board/ryco/configs: add raspberry pi project configs