Change kernel vboot header layout and add support for separate header verification.

This CL adds 2 things:
- Instead of having a kernel config, now we have a kernel preamble which contains some important parameters needed by the bootloader in the firmware to kernel hand-off. These parameters are verified using a separate preamble signature in addition to the kernel signature on actual kernel image data.

- Adds a new VerifyKernelHeader() API function which verifies the kernel verified boot header excluding the kernel data and also extracts parameters out of this header needed to verify the actual kernel image data (if deemed necessary). This allows for vboot header verification and data verification to be performed separately.

Review URL: http://codereview.chromium.org/2234003
7 files changed