bitbake.conf: Add runtimedir

bitbake.conf: Add runtimedir

Adds the path to the runtime state directory (/run). In particular,
systemd units need to have the correct path to the runtime directory
because RequiresMountsFor doesn't follow symbolic links. This means that
if a unit calls out a directory in /var/run (a symbolic link to /run),
it may actually start before /run is mounted. The fix is to have the
unit specify a directory in /run instead.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>