labs: kernel: setup: Clarify the flashing instructions

Signed-off-by: Miquel Raynal <[email protected]>

labs/kernel-board-setup/kernel-board-setup.tex

@@ -109,35 +109,40 @@ \section{Setting up serial communication with the board}
 
 \section{Bootloader interaction}
 
-Reset your board. Press the space bar in the \code{picocom} terminal
-to stop the U-boot countdown. You should then see the U-Boot prompt:
+In order to follow the labs, {\bf you need to use a board flashed
+with a specific Bootloader!}
 
-\begin{verbatim}
-=>
-\end{verbatim}
+If you are in an on-site session and the trainer gave you the hardware,
+the right Bootloader is already there, you just need to reset the
+environment (see below).
 
-You can now use U-Boot. Run the \code{help} command to see the available
-commands.
+If you are doing the labs on your own, please follow instructions 1. and
+2a. from the README available at:
+\url{https://github.com/bootlin/training-materials/tree/master/lab-data/common/bootloader/beaglebone-black}
 
-Type the \code{help saveenv} command to make sure that the
-\code{saveenv} command exists. We use it in these labs to
-save your U-Boot environment settings to the boards' eMMC storage.
-Some earlier versions do not support this.
+Do not try to use stock/random images from the Internet!
 
-If you don't have this U-Boot prompt, it's probably because you are doing these labs on your own
-(i.e. without participating to a Bootlin course), we ask you to install the U-Boot binary
-that we compiled and tested. See instructions at
-\url{https://github.com/bootlin/training-materials/tree/master/lab-data/common/bootloader/beaglebone-black}
-for a simple way to do this.
+Once this done, reset your board, press the space bar in the
+\code{picocom} terminal to stop the U-boot countdown. You should then
+see the U-Boot prompt:
 
-To avoid trouble because of settings applied in previous practical labs,
-we advise you to clear the U-Boot environment variables:
+\begin{verbatim}
+=>
+\end{verbatim}
+
+Then reset the U-Boot environment variables:
 
 \begin{verbatim}
 env default -f -a
 saveenv
 \end{verbatim}
 
+If the \code{saveenv} command fails, it means you need to follow
+(again?) the flashing procedure above.
+
+Otherwise, you can now use U-Boot! Type \code{help} to see the available
+commands.
+
 \section{Setting up networking}
 
 The next step is to configure U-boot and your workstation to let your