dev-manual, mega-manual: Updated kernel flow (text and figs)

Partial fix for [YOCTO #5368]

Updated the figure to not have the tarball method in there
for setting up poky repo locally.  Also, added specific references
to the environment setup scripts.

(From yocto-docs rev: 7c6ececaf8eb987206567077f42213dbe4cdc4f2)

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

documentation/dev-manual/dev-manual-model.xml

@@ -418,8 +418,9 @@
                         If you have never built the kernel you are interested in, you need to run
                         an initial build to establish local kernel source files.</para>
                         <para>If you are building an image for the first time, you need to get the build
-                        environment ready by sourcing
-                        the environment setup script.
+                        environment ready by sourcing an environment setup script
+                        (e.g. <filename>oe-init-build-env</filename> or
+                        <filename>oe-init-build-env-memres</filename>).
                         You also need to be sure two key configuration files
                         (<filename>local.conf</filename> and <filename>bblayers.conf</filename>)
                         are configured appropriately.</para>