Add VBOOT to top-level Makefile

This CL adds argument "VBOOT" to top-level Makefile and config.mk that points
to a verify boot implementation.

Change-Id: Ib468cc7a6a7c3ce3db56b90d73c6022ac30fa046

BUG=none
TEST=Run "make all" successfully

Review URL: http://codereview.chromium.org/3515011
2 files changed