write_tegra_bios replaces the numerous burn-u-boot scripts.

This tool uses the overlay list to find the board specific
files needed to write a new BIOS image to a tegra board.

Change-Id: I31793062c383ffa351b76b80882f8d69c2fce560

BUG=None
TEST=flash U-Boot on all tegra boards (with some changes to the burn-u-boot directories).

Review URL: http://codereview.chromium.org/6374016
tree: 4b825dc642cb6eb9a060e54bf8d69288fbee4904

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