board/stm32mp157c-dk2/genimage.cfg: drop hardcoded rootfs partition size

Let genimage to match the size of the rootfs image instead.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/stmicroelectronics/stm32mp157c-dk2/genimage.cfg

@@ -19,6 +19,5 @@ image sdcard.img {
 		image = "rootfs.ext4"
 		partition-type = 0x83
 		bootable = "yes"
-		size = 256M
 	}
 }