vboot: provide CHIPSET_PROVIDES_VERSTAGE_MAIN_SYMBOL option

Certain chipsets provide their own main symbol for verstage.
Therefore, it's necessary to know this so that those chipsets
can leverage the common verstage flow.

BUG=chrome-os-partner:44827
BRANCH=None
TEST=Built nyan using this option.

Change-Id: If80784aa47b27f0ad286babcf0f42ce198b929e9
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/11777
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/vendorcode/google/chromeos/vboot2/Kconfig

@@ -65,6 +65,13 @@ config RETURN_FROM_VERSTAGE
 	  reused by the succeeding stage. This is useful if a ram space is too
 	  small to fit both the verstage and the succeeding stage.
 
+config CHIPSET_PROVIDES_VERSTAGE_MAIN_SYMBOL
+	bool "The chipset provides the main() entry point for verstage"
+	default n
+	depends on SEPARATE_VERSTAGE
+	help
+	  The chipset code provides their own main() entry point.
+
 # These VBOOT_X_INDEX are the position of X in FW_MAIN_A/B region. The index
 # table is created by cros_bundle_firmware at build time based on the positions
 # of the blobs listed in fmap.dts and stored at the top of FW_MAIN_A/B region.

src/vendorcode/google/chromeos/vboot2/verstage.c

@@ -42,5 +42,7 @@ void verstage(void)
 	}
 }
 
+#if !IS_ENABLED(CONFIG_CHIPSET_PROVIDES_VERSTAGE_MAIN_SYMBOL)
 /* This is for boards that rely on main() for an entry point of a stage. */
 void main(void) __attribute__((alias ("verstage")));
+#endif