Fix and unify the commands creating the link target of /dev/shm

$(realpath /dev/shm) will return the absolute path of the target of
/dev/shm, thus the command will work for both absolute symlink and
relative symlink.

chapter07/kernfs.xml

@@ -97,7 +97,8 @@ mount -vt tmpfs tmpfs $LFS/run</userinput></screen>
     </variablelist>
 -->
       <para>In some host systems, <filename>/dev/shm</filename> is a
-      symbolic link to <filename class="directory">/run/shm</filename>.
+      symbolic link to a directory, typically
+      <filename class="directory">/run/shm</filename>.
       The /run tmpfs was mounted above so in this case only a
       directory needs to be created.</para>
 
@@ -107,7 +108,7 @@ mount -vt tmpfs tmpfs $LFS/run</userinput></screen>
       we must explicitly mount a tmpfs:</para>
 
 <screen><userinput>if [ -h $LFS/dev/shm ]; then
-  (cd $LFS/dev; mkdir $(readlink shm))
+  mkdir -pv $LFS$(realpath /dev/shm)
 else
   mount -vt tmpfs -o nosuid,nodev tmpfs $LFS/dev/shm
 fi</userinput></screen>

chapter11/afterlfs.xml

@@ -130,7 +130,7 @@ mounttype proc    proc   proc
 mounttype sys     sysfs  sysfs
 mounttype run     tmpfs  run
 if [ -h $LFS/dev/shm ]; then
-  mkdir -pv $LFS/$(readlink $LFS/dev/shm)
+  mkdir -pv $LFS$(realpath /dev/shm)
 else
   mounttype dev/shm tmpfs tmpfs -o nosuid,nodev
 fi