samples: ipc: openamp: update build documentation

Update build documentation for openamp sample, to reflect that sysbuild
is required when building this sample. Sysbuild is needed to build both
core images in one step.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>

samples/subsys/ipc/openamp/README.rst

@@ -9,7 +9,7 @@ Overview
 This application demonstrates how to use OpenAMP with Zephyr. It is designed to
 demonstrate how to integrate OpenAMP with Zephyr both from a build perspective
 and code. Note that the remote and primary core images can be flashed
-independently, but sysbuild must be used in order to flash them in one step.
+independently, but sysbuild must be used in order to build the images.
 
 Building the application for lpcxpresso54114_m4
 ***********************************************
@@ -18,6 +18,7 @@ Building the application for lpcxpresso54114_m4
  :zephyr-app: samples/subsys/ipc/openamp
  :board: lpcxpresso54114_m4
  :goals: debug
+ :west-args: --sysbuild
 
 Building the application for lpcxpresso55s69_cpu0
 *************************************************
@@ -26,6 +27,7 @@ Building the application for lpcxpresso55s69_cpu0
  :zephyr-app: samples/subsys/ipc/openamp
  :board: lpcxpresso55s69_cpu0
  :goals: debug
+ :west-args: --sysbuild
 
 Building the application for mps2_an521
 ***************************************
@@ -34,6 +36,7 @@ Building the application for mps2_an521
  :zephyr-app: samples/subsys/ipc/openamp
  :board: mps2_an521
  :goals: debug
+ :west-args: --sysbuild
 
 Building the application for v2m_musca_b1
 *****************************************
@@ -42,6 +45,7 @@ Building the application for v2m_musca_b1
  :zephyr-app: samples/subsys/ipc/openamp
  :board: v2m_musca_b1
  :goals: debug
+ :west-args: --sysbuild
 
 Building the application for mimxrt1170_evk_cm7
 ***********************************************
@@ -50,6 +54,7 @@ Building the application for mimxrt1170_evk_cm7
  :zephyr-app: samples/subsys/ipc/openamp
  :board: mimxrt1170_evk_cm7
  :goals: debug
+ :west-args: --sysbuild
 
 Open a serial terminal (minicom, putty, etc.) and connect the board with the
 following settings: