imx.mk: allow to override OPTEE_OS_PLATFORM

The makefile can be reused for other imx platforms.
Start to decouple from imx-mx8mqevk and avoid to hardcode.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/imx.mk b/imx.mk
index a5e76fc..1b77160 100644
--- a/imx.mk
+++ b/imx.mk
@@ -8,7 +8,7 @@
 override COMPILE_S_USER    := 64
 override COMPILE_S_KERNEL  := 64
 
-OPTEE_OS_PLATFORM = imx-mx8mqevk
+OPTEE_OS_PLATFORM ?= imx-mx8mqevk
 BR2_TARGET_GENERIC_GETTY_PORT = ttymxc0
 BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y