download/u-boot: Support reading tree and revision from board.cfg

Boards may want to specify a board-specific U-Boot revision. At the very
least, pseudo-boards for u-boot-libre releases will need to specify their
U-Boot versions somehow.

Copy the existing mechanism from download/coreboot for specifying
build info with board.cfg files. Specify the commit hash for the
'v2021.07' pseudo-board, and 'master' as the default.

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