imx.mk: introduce TFA_PLATFORM

Similar to OPTEE_OS_PLATFORM variable, allow to override so the makefile
becomes a bit more generic.

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 1b77160..96fdbd5 100644
--- a/imx.mk
+++ b/imx.mk
@@ -8,6 +8,7 @@
 override COMPILE_S_USER    := 64
 override COMPILE_S_KERNEL  := 64
 
+TFA_PLATFORM      ?= imx8mq
 OPTEE_OS_PLATFORM ?= imx-mx8mqevk
 BR2_TARGET_GENERIC_GETTY_PORT = ttymxc0
 BR2_TARGET_ROOTFS_EXT2=y
@@ -51,7 +52,7 @@
 #	BL32_EXTRA1=$(OPTEE_OS_PAGER_V2_BIN) \
 #	BL32_EXTRA2=$(OPTEE_OS_PAGEABLE_V2_BIN) \
 
-TF_A_FLAGS  = PLAT=imx8mq SPD=opteed DEBUG_CONSOLE=1 DEBUG=0 V=1
+TF_A_FLAGS  = PLAT=$(TFA_PLATFORM) SPD=opteed DEBUG_CONSOLE=1 DEBUG=0 V=1
 TF_A_FLAGS += BL32=$(ROOT)/optee_os/out/arm/core/tee-raw.bin
 
 tfa: optee-os u-boot