add SetVirtualDevMode stub

Fixes build failure seen in u-boot for tegra2 seaboard:
.../vboot_fw.a(vboot_api_kernel.o): In function 'VbConfirmChangeDevMode':
(.text+0x84): undefined reference to 'SetVirtualDevMode'

BUG=None
TEST=`emerge-tegra2_seaboard chromeos-u-boot` works again

Change-Id: If374a78218fc8a7921ff8dddde163138969e0876
Reviewed-on: https://gerrit.chromium.org/gerrit/24948
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>

firmware/lib/mocked_rollback_index.c

@@ -11,6 +11,11 @@
 #include "tss_constants.h"
 
 
+uint32_t SetVirtualDevMode(int val) {
+  return TPM_SUCCESS;
+}
+
+
 uint32_t TPMClearAndReenable(void) {
   return TPM_SUCCESS;
 }