flapjack: Support getting sku_id from FDT

On Flapjack, coreboot would read sku id form CBI and export it into the
device tree. Please refer to CL:1494093

So add read sku_id support for kukui & flapjack.

BUG=b:126805532
TEST=1:) emerge-flapjack mosys;
     2:) run 'mosys platform sku' in flapjack shows the string format for
     /proc/device-tree/firmware/coreboot/sku-id

Change-Id: Ia4d402e25cead5d1ff8bb571188aa3821170b106
Reviewed-on: https://chromium-review.googlesource.com/1496677
Commit-Ready: Kaka Ni <nigang@huaqin.corp-partner.google.com>
Tested-by: Kaka Ni <nigang@huaqin.corp-partner.google.com>
Reviewed-by: YH Lin <yueherngl@chromium.org>
3 files changed