arm-trusted-firmware: simplify release dir path

The path to the binary images is very long. Since we are about to make
a larger use of it, let's use a variable to make it somewhat shorter.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/arm-trusted-firmware/arm-trusted-firmware.mk

@@ -25,6 +25,7 @@ endif
 ARM_TRUSTED_FIRMWARE_INSTALL_IMAGES = YES
 
 ARM_TRUSTED_FIRMWARE_PLATFORM = $(call qstrip,$(BR2_TARGET_ARM_TRUSTED_FIRMWARE_PLATFORM))
+ARM_TRUSTED_FIRMWARE_IMG_DIR = $(@D)/build/$(ARM_TRUSTED_FIRMWARE_PLATFORM)/release
 
 ARM_TRUSTED_FIRMWARE_MAKE_OPTS += \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
@@ -82,7 +83,7 @@ define ARM_TRUSTED_FIRMWARE_BUILD_CMDS
 endef
 
 define ARM_TRUSTED_FIRMWARE_INSTALL_IMAGES_CMDS
-	cp -dpf $(@D)/build/$(ARM_TRUSTED_FIRMWARE_PLATFORM)/release/*.bin $(BINARIES_DIR)/
+	cp -dpf $(ARM_TRUSTED_FIRMWARE_IMG_DIR)/*.bin $(BINARIES_DIR)/
 endef
 
 # Configuration check