Mounting /dev/shm can break some hosts

[Matthew Burgess]

As reported at ​http://www.linuxfromscratch.org/pipermail/lfs-dev/2012-December/067476.html the instructions to work around the fact that some hosts have symlinked /dev/shm to /run/shm can in fact break those hosts. Once the /dev/shm symlink is removed from the host, it's never recreated!

The fix from Pierre, in that thread, looks good:

if [ -h $LFS/dev/shm ]; then
   mkdir -p $/LFS/run/shm
   mount -vt tmpfs shm $LFS/run/shm
fi

So, we mount the shm FS into the target of the symlink, rather than removing the symlink.

We'll need to add an errata item for this as well.