board/olimex/a20_olinuxino/genimage.cfg: fix option suffix

With commit[1] I've introduced a code style bug:
offset = 8KB
so let's substitute 8KB with 8K.

[1]: https://gitlab.com/buildroot.org/buildroot/-/commit/43bfa1cf1046b0e44e87a02bc5a358bf1de5bfdd

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/olimex/a20_olinuxino/genimage.cfg

@@ -5,7 +5,7 @@ image sdcard.img {
 	partition u-boot {
 		in-partition-table = "no"
 		image = "u-boot-sunxi-with-spl.bin"
-		offset = 8KB
+		offset = 8K
 		size = 1016K # 1MB - 8KB
 	}