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>
1 file changed