doc: up_squared: add 'west' build option to the UP2 documentation

It is also possible to use 'west' to build the 'hello_world' sample
application for the UP2 (up_squared) board. This patch makes it
explicit in the documentation.

Signed-off-by: Geoffroy Van Cutsem <[email protected]>

Author: Geoffroy Van Cutsem

boards/x86/up_squared/doc/index.rst

@@ -208,6 +208,7 @@ Build Zephyr application
 
    .. zephyr-app-commands::
       :zephyr-app: samples/hello_world
+      :tool: all
       :board: up_squared
       :goals: build