google/oak: Add derivative board Elm

This patch adds a new mainboard Google/Elm as a derivative of
Google/Oak, using the same code sharing technique for derivative boards
that was pioneered with Google/Veyron*. For now, there are no
firmware-relevant fundamental differences between the two boards.

In addition, introduce a board-specific Kconfig for the "board ID
adjustment" to represent the fact that the Elm board ID space mirrors
the Oak board ID space with an offset of 6, meaning Elm rev0 is
equivalent to Oak rev6, and future board changes will be made on both
boards to maintain this stride (at least virtually... not all of those
revisions will necessarily get built). This should make it much easier
to keep the code that handles revision differences somewhat clean.
(That's the theory, anyway... whether it will work out remains to be
seen.)

BRANCH=None
BUG=None
TEST=Booted Elm image with hardcoded board ID 0 on Oak rev6.

Change-Id: If540aea862b746cf4986a74482ae1764c104fb73
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 53cd85c94945ab0bf14cb88a98e66723fc4483de
Original-Change-Id: Ib05fc81dc4f4308d99e34fce74c6db8b323785da
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/332276
Original-Commit-Ready: Yidi Lin <yidi.lin@mediatek.com>
Original-Tested-by: Yidi Lin <yidi.lin@mediatek.com>
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/14691
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/mainboard/google/oak/chromeos.c

@@ -31,7 +31,7 @@ void setup_chromeos_gpios(void)
 	gpio_input_pullup(EC_IRQ);
 	gpio_input_pullup(LID);
 	gpio_input_pullup(POWER_BUTTON);
-	if (board_id() < 5)
+	if (board_id() + CONFIG_BOARD_ID_ADJUSTMENT < 5)
 		gpio_output(EC_SUSPEND_L, 1);
 }