[nrf fromlist] sysbuild: move sysbuild/application to _sysbuild

The sysbuild part of sysbuild/application creates a sysbuild/application
folder under top-level build dir. However, having a sysbuild folder at
this level means that tab completion for sysbuild pre-fixed build
targets will stop after completing just `sysbuild` and then show several
subfolders, making it hard to see important build targets, such as:
`ninja sysbuild_menuconfig`.

As `sysbuild/application` is just a folder name, then move it to
`_sysbuild` instead to avoid impacting tab completion for  ninja and
make.

Also strip the `application` part in the process, as the extra folder
doesn't provide any extra benefit, but is just one more level to enter
when browsing the build folder structure.

Upstream PR: zephyrproject-rtos/zephyr#74210

Signed-off-by: Torsten Rasmussen <[email protected]>

share/sysbuild/CMakeLists.txt

@@ -21,7 +21,7 @@ if(NOT DEFINED APPLICATION_BINARY_DIR)
 endif()
 
 if(EXISTS ${APP_DIR}/sysbuild/CMakeLists.txt)
-  add_subdirectory(${APP_DIR}/sysbuild sysbuild/application)
+  add_subdirectory(${APP_DIR}/sysbuild _sysbuild)
 else()
-  add_subdirectory(template sysbuild/application)
+  add_subdirectory(template _sysbuild)
 endif()