systemd: Fix udevd init script so sysvinit in systemd mode works

Commit http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=f76d4b3549ca220fa4bf84db2756ab45e11d06a3
moved volatiles handling for /run to the udev code only. This breaks
sysvinit+systemd combined systems when building sysvinit images.

This patch hacks the udevd init script in systemd to provide
the missing symlink allowing the sysvinit images built with udevd
from systemd to work correctly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>