board/nezha/genimage.cfg: fix offset value notation

Option "offset = 12288K" must be "offset = 12M" according to the new
guidelines specified in Buildroot Manual with previous patch.

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

board/nezha/genimage.cfg

@@ -19,7 +19,7 @@ image sdcard.img {
 	partition u-boot-1 {
 		in-partition-table = "no"
 		image = "u-boot.toc1"
-		offset = 12288K
+		offset = 12M
 	}
 
 	partition u-boot-2 {