manager: fix KsuCli cmd userspace: reuse choose_boot_device - manager: simplify find boot image Co-authored-by: weishu <twsxtd@gmail.com> Co-authored-by: YuKongA <70465933+YuKongA@users.noreply.github.com> Co-authored-by: ShirkNeko <109797057+ShirkNeko@users.noreply.github.com>