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>
diff --git a/board/toradex/colibri_t20_iris/Makefile b/board/toradex/colibri_t20_iris/Makefile
index ebeac70..f286aec 100644
--- a/board/toradex/colibri_t20_iris/Makefile
+++ b/board/toradex/colibri_t20_iris/Makefile
@@ -4,6 +4,6 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	:= ../../nvidia/common/board.o
-obj-y	+= ../colibri_t20-common/colibri_t20-common.o
+include $(srctree)/board/nvidia/common/common.mk
+
 obj-y	+= colibri_t20_iris.o