google/chromeec: Add common infrastructure for boot-mode switches

Instead of defining the same functions for reading/clearing boot-mode
switches from EC in every mainboard, add a common infrastructure to
enable common functions for handling boot-mode switches if
GOOGLE_CHROMEEC is being used.

Only boards that were not moved to this new infrastructure are those
that do not use GOOGLE_CHROMEEC or which rely on some mainboard specific
mechanism for reading boot-mode switches.

BUG=None
BRANCH=None
TEST=abuild compiles all boards successfully with and without ChromeOS
option.

Change-Id: I267aadea9e616464563df04b51a668b877f0d578
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/17449
Tested-by: build bot (Jenkins)
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/mainboard/intel/strago/Kconfig

@@ -16,6 +16,7 @@ config BOARD_SPECIFIC_OPTIONS
 	select PCIEXP_L1_SUB_STATE
 
 config CHROMEOS
+	select EC_GOOGLE_CHROMEEC_SWITCHES
 	select EC_SOFTWARE_SYNC
 	select LID_SWITCH
 	select VBOOT_DYNAMIC_WORK_BUFFER