mb/google/poppy/variants/nami: change type of board_sku_id() to uint32_t

Tools/scripts, like mosys/arc-setup, use int (4 bytes) to
read the sku id. In order to support "-1", we need to use
uint32_t (4 bytes) instead of using uint16_t (2 bytes) data type.
Otherwise, tools/scripts will read 65535 instead of -1.
Another reason to change this is that sku_id can be
supported by ec up to 4 bytes.

TEST=mosys output "Platform not supported" for -1 sku id
     arc-setup read -1 sku id

Change-Id: Ib3baa8419f138abeb412ac09c2e7dc608e3b758b
Signed-off-by: Zhuohao Lee <zhuohao@chromium.org>
Reviewed-on: https://review.coreboot.org/25252
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
4 files changed