ARM: tegra: set env vars to indicate Cardhu A04 support

The U-Boot "cardhu" build supports only revision 4 of the Cardhu board
and later compatible revisions. Hence, set $board_name in the default
environment to "cardhu-a04" rather than just "cardhu".

The Linux kernel has separate DTs for Cardhu A02 and A04, although the
former isn't really supported any more. Consequently, the kernel DT file
that matches the U-Boot cardhu build is "tegra30-cardhu-a04.dtb" rather
than "tegra30-cardhu.dtb". Set the $fdtfile default environment variable
to reflect this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>