ARM: tegra: get rid of colibri_t20-common

As a preparatory step to renaming the board folder as well first get
rid of the colibri_t20-common after having integrated it into
colibri_t20_iris for now.

While at it also migrate to using NVIDIA's common.mk magic.

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