image_types_wic.bbclass: alphabetize list of WICVARS

It's way easier to scan a list of variables if they're sorted one per
line.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>