Zero-out allocated buffer while creating kernel preamble header.

Zero-out allocated buffer while creating kernel preamble header.

Since the kernel vblocks are always padded out to a fixed size, the unused (padded) memory was ending up containing random bytes, leading to vbutil_kernel generating vblocks that are not bit-identical when run with the same inputs.

BUG=none
TEST=see below
1) Use vbutil_kernel with the same set of inputs to generate two alternative vblocks.
2) Compare the 2 files - they must be bitwise identical space. The padding bytes must all be set to 0.
3) Generate a new signed image using the resign_image.sh script under scripts/image_signing. This signed image should be boot and install successfully on a maching running our custom firmware using the same set of root, firmware, and kernel keys.

[Bill Richardson, 2010-08-04 23:40:40]

LGTM