Tegra: T30: Beaver: Fix board/board_name env vars, s/b beaver, not cardhu

Did a 'strings u-boot-dtb-tegra.bin | less' and saw that both
board and board_name == beaver. Didn't test as I have no T30
Beaver board here.

Signed-off-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>