labs: yocto: beaglebone: update lab6 to scarthgap

* Switch from linux-ti-staging to linux-bb.org. Same for U-boot.
* The linux-bb.org and u-boot-bb.org recipes are setting the
  COMPATIBLE_MACHINE variable, so we need a way to also be compatible
  with these recipes. For that, add a custom override to
  MACHINEOVERRIDES.
* The default config does not generate tar.xz rootfs by default, add an
  instruction to do that.

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

labs/yocto-custom-machine/yocto-custom-machine.tex

@@ -101,12 +101,19 @@ \section{Populate the machine configuration}
 \end{verbatim}
   \else
 \begin{itemize}
-  \item To select \code{linux-ti-staging} as the preferred provider
+  \item To select \code{linux-bb.org} as the preferred provider
     for the kernel.
   \item To build \code{am335x-boneblack.dtb} and the
     \code{am335x-boneblack-wireless.dtb} device trees.
-  \item To select \code{u-boot-ti-staging} as the preferred provider
+  \item To select \code{u-boot-bb.org} as the preferred provider
     for the bootloader.
+  \item To be compatible with the \code{linux-bb.org} and \code{u-boot-bb.org}
+    recipes, which by default are designed to only be compatible with the
+    BeagleBone machines. To make our \code{bootlinlabs} machine compatible with
+    these recipes, add the string \code{:beaglebone} to the
+    \code{MACHINEOVERRIDES} variable\footnote{The underlying mechanism that the
+    \code{linux-bb.org} and \code{u-boot-bb.org} recipes use is the
+    \yoctovar{COMPATIBLE_MACHINE} mechanism.}.
   \item To use \code{arm} as the U-Boot architecture.
   \item To use \code{am335x_evm_config} as the U-Boot
     configuration target.
@@ -123,6 +130,7 @@ \section{Populate the machine configuration}
       \item \code{ext2}
       \item \code{alsa}
     \end{itemize}
+  \item To add \code{tar.xz} as a rootfs type to generate.
   \fi
 \fi
 \end{itemize}