Incorrect default chip SKU for Orin Nano targets

[madisongh]

This came up in #1655 -

The default chip SKU is being set on a per-board basis. Starting with L4T R35.5.0, the chip SKU is used to select the BPMP firmware to use. The problem is that the P3767 boards, the default chip SKU is D3, which selects the TE980M firmware, but the Orin Nano needs the TE950M firmware.

When flashing this isn't a big deal, since the actual chip SKU gets read from the target. But when signing during the build, only defaults are used, so we end up with the wrong pre-signed firmware being loaded onto the target.

We should probably address this by adding a TEGRA_CHIP_SKU variable that can be set in the machine configuration file as a default to be passed to the helper scripts.