boardid: Add helpers to read sku_id strapping into coreboot tables

On many recent Chrome OS boards, the desire to unite more configurations
under the same image has led to the need of a "SKU ID" that identifies
different configurations of the same board (e.g. with certain optional
components stuffed or not stuffed, or replaced with a comparable
component). This is markedly different from the existing "board ID",
because that is reserved to count "revisions" -- changes made to the
same configuration over time during the development process. This patch
adds support to have a mainboard define this SKU ID and pass it through
the coreboot table like we already have for board IDs.

Change-Id: I8aabffe8e1003b0d6fb70d689ae513ca4b46aeda
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/22696
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/boardid.h

@@ -20,7 +20,8 @@
 
 #define UNDEFINED_STRAPPING_ID (~0)
 
-uint32_t board_id(void);
-uint32_t ram_code(void);
+uint32_t board_id(void);	/* differentiates revisions */
+uint32_t ram_code(void);	/* identifies installed DRAM modules */
+uint32_t sku_id(void);		/* differentiates other optional components */
 
 #endif /* __INCLUDE_BOARDID_H__ */