slides/yocto-recipe-advanced: add slide on using classes

Clarify the difference between the recipe and global classes by showing
the are used, either with `inherit` or `INHERIT`.

Signed-off-by: Antonin Godard <[email protected]>

slides/yocto-recipe-advanced/yocto-recipe-advanced.tex

@@ -121,21 +121,32 @@ \subsection{Classes}
     \item Classes extension is \code{.bbclass}
     \item Classes must be located in the \code{classes-recipe},
       \code{classes-global}, or \code{classes} folders of a layer.
-    \item Recipes can use this common code by inheriting a class:
+  \end{itemize}
+\end{frame}
+
+\begin{frame}
+  \frametitle{Using classes}
+  \begin{itemize}
+    \item Recipes can use the classes located in the \code{classes-recipe}
+      folder:
     \begin{itemize}
-      \item \code{inherit <class>}
+      \item \code{inherit class1 class2 ...}
     \end{itemize}
     \item A recipe can inherit from multiple classes.
+    \item Classes in \code{classes-global} can be inherited from configuration
+      files with \yoctovar{INHERIT}:
+    \begin{itemize}
+      \item \code{INHERIT:append = " class1 class2"}
+    \end{itemize}
+    \item Classes in \code{INHERIT} will be used by every built recipe.
+    \item Classes in the \code{classes} folder can be used with \code{inherit} or
+      \yoctovar{INHERIT}, as their usage is not clearly defined.
   \end{itemize}
 \end{frame}
 
 \begin{frame}
   \frametitle{Common classes}
   \begin{itemize}
-    \item Most classes are split into scope-specific subdirectories:
-      \code{meta/classes-recipe/}, \code{meta/classes-global/}
-    \item Classes whose usage context is not clearly defined can be found in
-      \code{meta/classes/}
     \begin{itemize}
       \item \code{classes-global/base.bbclass}
       \item \code{classes-recipe/kernel.bbclass}