vboot: Remove CHIPSET_PROVIDES_VERSTAGE_MAIN_SYMBOL Kconfig option

CHIPSET_PROVIDES_VERSTAGE_MAIN_SYMBOL allows the SoC directory to
provide its own main() symbol that can execute code before the generic
verstage code runs. We have now established in other places (e.g. T210
ramstage) a sort of convention that SoCs which need to run code in any
stage before main() should just override stage_entry() instead. This
patch aligns the verstage with that model and gets rid of the extra
Kconfig option. This also removes the need for aliasing between main()
and verstage(). Like other stages the main verstage code is now just in
main() and can be called from stage_entry().

Change-Id: If42c9c4fbab51fbd474e1530023a30b69495d1d6
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/18978
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/arch/x86/verstage.c

@@ -14,10 +14,10 @@
  */
 
 #include <arch/cpu.h>
-#include <vendorcode/google/chromeos/chromeos.h>
+#include <main_decl.h>
 
 /* Provide an entry point for verstage when it's a separate stage. */
 asmlinkage void car_stage_entry(void)
 {
-	verstage();
+	main();
 }