imx.mk: allow to pass additional arguments to TF_A_FLAGS

Some boards require to pass additional variables like IMX_BOOT_UART_BASE
and ERRATAs.

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 515bffd..d179b49 100644
--- a/imx.mk
+++ b/imx.mk
@@ -54,7 +54,7 @@
 #	BL32_EXTRA1=$(OPTEE_OS_PAGER_V2_BIN) \
 #	BL32_EXTRA2=$(OPTEE_OS_PAGEABLE_V2_BIN) \
 
-TF_A_FLAGS  = PLAT=$(TFA_PLATFORM) SPD=opteed DEBUG_CONSOLE=1 DEBUG=0 V=1
+TF_A_FLAGS += PLAT=$(TFA_PLATFORM) SPD=opteed DEBUG_CONSOLE=1 DEBUG=0 V=1
 TF_A_FLAGS += BL32=$(OPTEE_OS_PATH)/out/arm/core/tee-raw.bin
 
 tfa: optee-os