slides/buildroot-advanced: add one slide about a few BR2_EXTERNAL trees

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

slides/buildroot-advanced/buildroot-advanced.tex

@@ -218,6 +218,27 @@ \section{Advanced topics}
 
 \end{frame}
 
+\begin{frame}{Examples of {\tt BR2\_EXTERNAL} trees}
+  \begin{itemize}
+  \item There are a number of publicly available {\tt BR2\_EXTERNAL}
+    trees, especially from hardware vendors:
+    \begin{itemize}
+    \item \code{buildroot-external-st}, maintained by Bootlin in
+      partnership with ST, containing example configurations for the
+      STM32MP1 platforms.\\
+      \url{https://github.com/bootlin/buildroot-external-st}
+    \item \code{buildroot-external-microchip}, containing example
+      configurations, additional packages and demo applications for
+      Microchip ARM platforms.\\
+      \url{https://github.com/linux4sam/buildroot-external-microchip}
+    \item \code{buildroot-external-boundary}, containing example
+      configurations for Boundary Devices boards, mainly based on NXP
+      i.MX processors.\\
+      \url{https://github.com/boundarydevices/buildroot-external-boundary}
+    \end{itemize}
+  \end{itemize}
+\end{frame}
+
 \begin{frame}{Package-specific targets: basics}
   \begin{itemize}
   \item Internally, each package is implemented through a number of