configs/nanopi_neo: determine roofs partition size automatically

Let genimage determine rootfs partition size based on the filesystem
image size.

Signed-off-by: Viktar Palstsiuk <viktar.palstsiuk@promwad.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/friendlyarm/nanopi-neo/genimage.cfg

@@ -29,6 +29,5 @@ image sdcard.img {
 	partition rootfs {
 		partition-type = 0x83
 		image = "rootfs.ext4"
-		size = 32M
 	}
 }