wic: engine.py: use raw string for escape sequence

Fixes:

| poky/scripts/lib/wic/engine.py:362: SyntaxWarning: invalid escape sequence '\/'

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e33d7241)
Signed-off-by: Steve Sakoman <steve@sakoman.com>