ARM: tegra: rename colibri_t20 board/configuration/device-tree

In accordance with our other modules supported by U-Boot and as agreed
upon for Apalis/Colibri T30 get rid of the carrier board in the board/
configuration/device-tree naming.

While at it also bring the prompt more in line with our other products.

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>