Add board_name to CONFIG_ENV_VARS_UBOOT_CONFIG

Add board_name to CONFIG_ENV_VARS_UBOOT_CONFIG

CONFIG_ENV_VARS_UBOOT_CONFIG creates environment variables indicating
which configuration U-Boot was built for. Some U-Boot binaries run on
multiple boards, and hence this information may not uniquley describe
the HW that U-Boot is actually running on. Another patch introduces
environment variable board_name to represent that. In order to avoid
scripts having to check $board_name, use it if set, and then fall back
to using $board, make CONFIG_ENV_VARS_UBOOT_CONFIG also set a default
value for board_name, so that variable is always available.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>