west: update build extension command to use APP_DIR

`west build` can be invoked without specifying the source directory when
being invoked from the source directory itself.
When using `west build` for incremental builds, then the build command
will examine the CMake cache to determine the application dir by using
the value of CMAKE_HOME_DIRECTORY.

With sysbuild, this leads to the wrong assumption that the sysbuild
itself is the application to build.

Instead, have west build look for APP_DIR which points to the correct
source dir when sysbuild is used. Use APPLICATION_SOURCE_DIR when
APP_DIR is not set, as this indicates a no-sysbuild build.

Keep CMAKE_HOME_DIRECTORY behavior as last fallback mechanism.

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