vboot: provide empty vboot_verify_firmware()

In the case of CONFIG_VBOOT_VERIFY_FIRMWARE not being
selected allow for calling vboot_verify_firmware()
with an empty implementation. This allows for one not to
clutter the source with ifdefs.

BUG=chrome-os-partner:23249
BRANCH=None
TEST=Built with a !CONFIG_VBOOT_VERIFY_FIRMWARE and non-guarded
     call to vboot_verify_firmware().

Change-Id: I72af717ede3c5d1db2a1f8e586fefcca82b191d5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/172711
Reviewed-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: http://review.coreboot.org/4879
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/cpu/intel/haswell/romstage.c

@@ -308,9 +308,7 @@ void romstage_after_car(void)
 
 	prepare_for_resume(handoff);
 
-#if CONFIG_VBOOT_VERIFY_FIRMWARE
 	vboot_verify_firmware(handoff);
-#endif
 
 	/* Load the ramstage. */
 	copy_and_run();