vboot: Provide declaration for verstage_mainboard_early_init()

Similar to bootblock, provide declaration for
verstage_mainboard_early_init() to support early mainboard
initialization if verstage is run before bootblock.

BUG=b:155824234
TEST=Verified that trembyle still builds

Signed-off-by: Furquan Shaikh <furquan@google.com>
Change-Id: I106213ecc1c44100f1f74071189518563ac08121
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41137
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/security/vboot/vboot_common.h

@@ -43,6 +43,7 @@ int vboot_retrieve_hash(void *digest, size_t digest_size);
  * If the verstage is a separate stage, it should be entered via main().
  */
 void verstage_main(void);
+void verstage_mainboard_early_init(void);
 void verstage_mainboard_init(void);
 
 /* Check boot modes */