imx.mk: remove tfa dependency on u-boot

TF-A build doesn't depends on U-Boot. It's the other way around.

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 3c5b467..7088174 100644
--- a/imx.mk
+++ b/imx.mk
@@ -53,7 +53,7 @@
 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
+tfa: optee-os
 	$(TF_A_EXPORTS) $(MAKE) -C $(TF_A_PATH) $(TF_A_FLAGS) all fip
 
 tfa-clean: