kernel-dev: Updated list of kernel repositories to have 4.12 and cache

I added the linux-yocto-4.12 kernel repository and the linux-yocto-cache
repository to the list of Yocto Linux Kernel repositories in the
Source Repositories.

(From yocto-docs rev: 88fedda0fa4a4dfc468f0dc404fa9f04390cbed7)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/kernel-dev/kernel-dev-concepts-appx.xml

@@ -81,6 +81,20 @@
                     Yocto Project Release 2.3.
                     This kernel is based on the Linux 4.10 released kernel.
                     </para></listitem>
+                <listitem><para>
+                    <emphasis><filename>linux-yocto-4.12</filename>:</emphasis>
+                    The default stable Yocto Project kernel to use with the
+                    Yocto Project Release 2.4.
+                    This kernel is based on the Linux 4.12 released kernel.
+                    </para></listitem>
+                <listitem><para>
+                    <emphasis><filename>yocto-kernel-cache</filename>:</emphasis>
+                    The <filename>linux-yocto-cache</filename> contains
+                    patches and configurations for the linux-yocto kernel
+                    tree.
+                    This repository is useful when working on the linux-yocto
+                    kernel.
+                    </para></listitem>
                 <listitem><para>
                     <emphasis><filename>linux-yocto-dev</filename>:</emphasis>
                     A development kernel based on the latest upstream release