imx.mk: allow the BR variables to be configured

They can be overriden if needed.

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 96fdbd5..a424fd4 100644
--- a/imx.mk
+++ b/imx.mk
@@ -10,9 +10,10 @@
 
 TFA_PLATFORM      ?= imx8mq
 OPTEE_OS_PLATFORM ?= imx-mx8mqevk
-BR2_TARGET_GENERIC_GETTY_PORT = ttymxc0
-BR2_TARGET_ROOTFS_EXT2=y
-BR2_TARGET_ROOTFS_EXT2_4=y
+
+BR2_TARGET_GENERIC_GETTY_PORT ?= ttymxc0
+BR2_TARGET_ROOTFS_EXT2 ?= y
+BR2_TARGET_ROOTFS_EXT2_4 ?= y
 
 include common.mk