cmake: sysbuild: sign: Fix skip size for nRF5340 network core

Fixes an issue with network core images for nRF5340 where there is
nothing to skip since the MCUboot header is applied externally
after the build by sysbuild

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

cmake/sysbuild/sign.cmake

@@ -104,6 +104,8 @@ function(b0_sign_image slot cpunet_target)
     set(skip_size 0)
   elseif(NOT SB_CONFIG_BOOTLOADER_MCUBOOT)
     set(skip_size 0)
+  elseif(cpunet_target)
+    set(skip_size 0)
   else()
     sysbuild_get(build_with_tfm IMAGE ${DEFAULT_IMAGE} VAR CONFIG_BUILD_WITH_TFM KCONFIG)
     if(build_with_tfm)