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/vendorcode/google/chromeos/chromeos.c

@@ -17,9 +17,15 @@
 #include <string.h>
 #include "chromeos.h"
 
+int __attribute__((weak)) get_developer_mode_switch(void)
+{
+	// Weak implementation. No physical developer switch.
+	return 0;
+}
+
 int __attribute__((weak)) clear_recovery_mode_switch(void)
 {
-	// Can be implemented by a mainboard
+	// Weak implementation. Nothing to do.
 	return 0;
 }