uboot: qstrip BR2_TARGET_UBOOT_CUSTOM_DTS_PATH

We need to avoid failing of copy of custom dts using cp -f <>
command. So, just qstrip the variable as is done for other
configuration options.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/uboot/uboot.mk

@@ -194,9 +194,11 @@ define UBOOT_HELP_CMDS
 endef
 endif # BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY
 
+UBOOT_CUSTOM_DTS_PATH = $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH))
+
 define UBOOT_BUILD_CMDS
-	$(if $(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH),
-		cp -f $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH)) $(@D)/arch/$(UBOOT_ARCH)/dts/
+	$(if $(UBOOT_CUSTOM_DTS_PATH),
+		cp -f $(UBOOT_CUSTOM_DTS_PATH) $(@D)/arch/$(UBOOT_ARCH)/dts/
 	)
 	$(TARGET_CONFIGURE_OPTS) 	\
 		$(MAKE) -C $(@D) $(UBOOT_MAKE_OPTS) 		\