buildroot

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>