Boot time labs: use the fdtargsaddr and fdtargslen variables

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

labs/boot-time-bootloader/boot-time-bootloader.tex

@@ -183,11 +183,12 @@ \section{Using U-Boot's {\em Falcon} mode}
 \end{verbatim}
 
 The last thing to do is to store such information in an \code{args} file
-in the FAT partition on the MMC, using the starting RAM address provided
-above and its size (\code{0x8ffeb3ff - 0x8ffd5000}):
+in the FAT partition on the MMC. Through the \code{fdtargsaddr} and
+\code{fdtargsaddr} environment variables, we know where to copy the
+data from and their size:
 
 \begin{verbatim}
-fatwrite mmc 0:1 0x8ffd5000 args 0x163ff
+fatwrite mmc 0:1 ${fdtargsaddr} args ${fdtargslen}
 \end{verbatim}
 
 You're ready to go and reboot your board with the SD card inside.