download/u-boot: Prepare files per board instead of per revision

The U-Boot download script is designed to help with releasing
u-boot-libre and it can only prepare a generic U-Boot v2021.07 tree.
However, we will need to build board-specific versions of U-Boot to be
able to use it as a coreboot payload effectively.

As a first step toward that, make the download script prepare per-board
copies of U-Boot v2021.07. Then, add a 'v2021.07' pseudo-board for the
u-boot-libre release script to work on.

The u-boot-libre deblob script hash ends up chaning due to copying my
author attribution from the download script, update its hash.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>