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().

Original-Change-Id: I72af717ede3c5d1db2a1f8e586fefcca82b191d5
Original-Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/172711
Original-Reviewed-by: Shawn Nematbakhsh <shawnn@chromium.org>
(cherry picked from commit c1e0e5c7b39c947b2a0c237b4678944ab86dd780)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Conflicts:
	src/vendorcode/google/chromeos/chromeos.h

Change-Id: Iaaa3bedbe8de701726c28412e7eb75de0c58c9c9
Reviewed-on: http://review.coreboot.org/7394
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>

src/vendorcode/google/chromeos/chromeos.h

@@ -34,11 +34,14 @@ void save_vbnv(const uint8_t *vbnv_copy);
 /* functions implemented in vboot.c */
 void init_chromeos(int bootmode);
 
+struct romstage_handoff;
 #if CONFIG_VBOOT_VERIFY_FIRMWARE
 /* Returns 0 on success < 0 on error. */
 int vboot_get_handoff_info(void **addr, uint32_t *size);
 int vboot_enable_developer(void);
 int vboot_enable_recovery(void);
+#else
+static inline void vboot_verify_firmware(struct romstage_handoff *h) {}
 #endif
 
 #include "gnvs.h"