package/busybox: add init script for sysctl

package/busybox: add init script for sysctl

Add a simple init script that invokes sysctl early in the initialization
process to configure kernel parameters. This is already performed by
systemd (systemd-sysctl) but there is no sysvinit/busybox counterpart.

Files are read from directories in the following list in the given order
from top to bottom:

    /run/sysctl.d/*.conf
    /etc/sysctl.d/*.conf
    /usr/local/lib/sysctl.d/*.conf
    /usr/lib/sysctl.d/*.conf
    /lib/sysctl.d/*.conf
    /etc/sysctl.conf

A file may be used more than once, since there can be multiple symlinks
to it. No attempt is made to prevent this.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>