documentation/bsp-guide/bsp.xml: Added new section

Fixes [YOCTO #1962]

Added a new section called "Customizing a Recipe for a BSP".
The text was rooted in Darren Hart's comments.  I implemented
them and then we iterated a bit on it.

Reported-by: Joshua Lock <joshua.lock@intel.com>
(From yocto-docs rev: f5e51d60312d9335a790023f193cae1ba76277ae)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/bsp-guide/bsp.xml

@@ -638,6 +638,42 @@
             </section>
         </section>
 
+        <section id='customizing-a-recipe-for-a-bsp'>
+            <title>Customizing a Recipe for a BSP</title>
+
+            <para>
+               If you plan on customizing a recipe for a particular BSP, you need to do the
+               following:
+               <itemizedlist>
+                   <listitem><para>Include within the BSP layer a <filename>.bbappend</filename>
+                       file for the modified recipe.</para></listitem>
+                   <listitem><para>Place the BSP-specific file in the BSP's recipe
+                       <filename>.bbappend</filename> file path under a directory named
+                       after the machine.</para></listitem>
+               </itemizedlist>
+           </para>
+
+           <para>
+               To better understand this, consider an example that customizes a recipe by adding 
+               a BSP-specific configuration file named <filename>interfaces</filename> to the 
+               <filename>netbase_4.47.bb</filename> recipe for machine "xyz".
+               Do the following:
+               <orderedlist>
+                   <listitem><para>Edit the <filename>netbase_4.47.bbappend</filename> file so that it 
+                       contains the following:
+                       <literallayout class='monospaced'>
+     FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
+     PRINC := "${@int(PRINC) + 2}"
+                       </literallayout></para></listitem>
+                   <listitem><para>Create and place the new <filename>interfaces</filename> 
+                       configuration file in the BSP's layer here:
+                       <literallayout class='monospaced'>
+     meta-xyz/recipes-core/netbase/files/xyz/interfaces
+                       </literallayout></para></listitem>
+               </orderedlist>
+            </para>
+        </section>
+
         <section id='bsp-licensing-considerations'>
             <title>BSP Licensing Considerations</title>