imx.mk: add imx8mp-evk and imx8mp-verdin board support

imx.mk is now generic to support other i.MX boards.
Add board specific config snippet as examples to support imx8mp-evk and
imx8mp-verdin platforms.
Keep imx.mk default behavior to support imx8mq-evk platform.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/imx.mk b/imx.mk
index 0ef7571..f3c022f 100644
--- a/imx.mk
+++ b/imx.mk
@@ -43,6 +43,14 @@
 
 BOOT_IMG		?= $(ROOT)/out/boot.img
 
+# Set the variable to include the board config snippet
+# Default value is empty, will select the imx8mq-evk board.
+# Possible values: "imx8mp-evk" or "imx8mp-verdin"
+IMX_BOARD ?=
+ifneq (,$(IMX_BOARD))
+include $(IMX_BOARD).inc.mk
+endif
+
 ################################################################################
 # Targets
 ################################################################################