labs/sysdev-kernel-cross-compiling: use \arch instead of \board

The logic to show the final U-Boot command used \board instead of
\arch to decide between bootz and booti, which worked fine when we had
just BeaglePlay as an ARM64 platform, but will fail with more ARM64
platforms supported. Fix that.

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

Author: tpetazzoni

labs/sysdev-kernel-cross-compiling/sysdev-kernel-cross-compiling.tex

@@ -254,7 +254,7 @@ \section{Load and boot the kernel using U-Boot}
 reset. Reset the board, and customize \code{bootcmd}:
 
 \begin{ubootinput}
-=> setenv bootcmd 'tftp %\zimageboardaddr\ \ifdefstring{\board}{beagleplay}{Image.gz}{zImage}; tftp \dtbboardaddr\ {\dtname}.dtb; \ifdefstring{\board}{beagleplay}{booti}{bootz} \zimageboardaddr\ - \dtbboardaddr'%
+=> setenv bootcmd 'tftp %\zimageboardaddr\ \ifdefstring{\board}{beagleplay}{Image.gz}{zImage}; tftp \dtbboardaddr\ {\dtname}.dtb; \ifdefstring{\arch}{ARM64}{booti}{bootz} \zimageboardaddr\ - \dtbboardaddr'%
 => saveenv
 \end{ubootinput}