kernfs: modernize the explanation for bind mounting /dev

If you are using a "modern" distro (with devtmpfs and a modern udev
implementation), a bind mounting is actually not needed because you can
mount devtmpfs anyway.  The only reason for bind mounting is to be
compatible with old host distros where /dev is a directory containing
many static device nodes, or is a tmpfs (not same as devtmpfs) popluated
by bootscript or an old udev (modern udev implementations, including
eudev and systemd-udev used by LFS, strictly requires a devtmpfs on
/dev).

So update the explanation to match the status quo.

chapter07/kernfs.xml

@@ -29,16 +29,27 @@
   <sect2 id="ch-tools-bindmount">
     <title>Mounting and Populating /dev</title>
 
-      <para>During a normal boot, the kernel automatically mounts the
+      <para>During a normal boot of the LFS system, the kernel automatically
-      <systemitem class="filesystem">devtmpfs</systemitem> filesystem on the
+      mounts the <systemitem class="filesystem">devtmpfs</systemitem>
-      <filename class="directory">/dev</filename> directory; the
+      filesystem on the
-      devices are created dynamically on that virtual filesystem when they
+      <filename class="directory">/dev</filename> directory; the kernel
-      are first detected or accessed. Device creation is generally done during the
+      creates device on that virtual filesystem during the boot process
-      boot process by the kernel and the udev program.
+      or when a device is first detected or accessed. The udev daemon may
-      Since the new system does not yet include udev and
+      change the owner or permission of the device nodes created by the
-      has not yet been booted, it is necessary to mount and populate
+      kernel, or create new device nodes or symlinks to ease the work of
-      the <filename class="directory">/dev</filename> directory manually. This is
+      distro maintainers or system administrators.  (See
-      accomplished by bind mounting the host system's
+      <xref linkend='ch-config-udev-device-node-creation'/> for details.)
+      If the host kernel supports &devtmpfs;, we can simply mount a
+      &devtmpfs; at <filename class='directory'>$LFS/dev</filename> and rely
+      on the kernel to populate it (the LFS building process does not need
+      the additional work onto &devtmpfs; by udev daemon).</para>
+
+      <para>But, some host kernels may lack &devtmpfs; support and these
+      host distros maintain the content of
+      <filename class="directory">/dev</filename> with different methods.
+      So the only host-agnostic way for populating
+      <filename class="directory">$LFS/dev</filename> is
+      bind mounting the host system's
       <filename class="directory">/dev</filename> directory. A bind mount is
       a special type of mount that allows you to create a mirror of a
       directory or mount point at some other location. Use the following

chapter09/udev.xml

@@ -93,7 +93,7 @@
 
     </sect3>
 
-    <sect3>
+    <sect3 id='ch-config-udev-device-node-creation'>
       <title>Device Node Creation</title>
 
       <para>Device files are created by the kernel by the <systemitem

general.ent

@@ -121,6 +121,7 @@
 
 <!ENTITY root            "<systemitem class='username'>root</systemitem>">
 <!ENTITY lfs-user        "<systemitem class='username'>lfs</systemitem>">
+<!ENTITY devtmpfs        "<systemitem class='filesystem'>devtmpfs</systemitem>">
 <!ENTITY fstab           "<filename>/etc/fstab</filename>">
 <!ENTITY boot-dir        "<filename class='directory'>/boot</filename>">
 <!ENTITY ch-final        "<xref linkend='chapter-building-system'/>">