pcduino: Accommodate U-Boot environment on SD-card

As described by http://linux-sunxi.org/Bootable_SD_card#SD_Card_Layout

The space betweem 544KB..1MB is reserved / used for the u-boot environment,
so mark it as such to make sure genimage doesn't put other partitions here.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/linksprite/pcduino/genimage.cfg

@@ -25,6 +25,7 @@ image sdcard.img {
 		in-partition-table = "no"
 		image = "u-boot-sunxi-with-spl.bin"
 		offset = 8192
+		size = 1040384 # 1MB - 8192
 	}
 
 	partition boot {