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.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>