board/pine64/pine64: use offset to specify boot partition alignment

Use the 'offset' property in genimage.cfg to align the boot partition on
1 MiB boundary. It is clearer than fixing the size of the previous
u-boot partition.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Mattia Narducci <mattianarducci1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/pine64/pine64/genimage.cfg

@@ -18,13 +18,13 @@ image sdcard.img {
 		in-partition-table = "no"
 		image = "u-boot-sunxi-with-spl.bin"
 		offset = 8K
-		size = 1016K # 1MB - 8KB
 	}
 
 	partition boot {
 		partition-type = 0xC
 		bootable = "true"
 		image = "boot.vfat"
+		offset = 1M
 	}
 
 	partition rootfs {