debugging: labs: add detail about accessing uboot console

Signed-off-by: Alexis Lothoré <[email protected]>

labs/debugging-setup/debugging-setup.tex

@@ -138,8 +138,10 @@ \section{U-Boot setup}
 
 In order to use a rootfs on NFS, we will use an external rootfs. This can be
 specified by passing bootargs to the kernel. To do so, we are going to set the
-\code{bootargs} U-Boot variable and save the environment. On the target, enter
-the following commands:
+\code{bootargs} U-Boot variable and save the environment. To be able to edit
+U-Boot variables, we need to interrupt its standard boot sequence: with the
+serial console opened, maintain Enter key while pressing once the reset button,
+and wait for U-Boot prompt to appear. Then, enter the following commands:
 
 \begin{bashinput}
 STM32MP1> env set bootargs root=/dev/nfs ip=192.168.0.100:::::eth0