tests/boot_linux_console: use os.path for filesystem paths

tests/boot_linux_console: use os.path for filesystem paths

Change extract_from_deb() to use os.path routines to manipulate the
filesystem path returned when extracting a file.

Suggested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1580914565-19675-7-git-send-email-liam.merwick@oracle.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>