u-boot: fix custom patch handling

When U-Boot is enabled and no custom patch directory has been set,
then the current test:

  ifneq ($(strip $(BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR)),"")

works. However, when U-Boot is not enabled, but still gets compiled
because mkimage is needed to build the kernel,
BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR is completely empty. It does not
even have quotes. So the test in fact needs to be:

 ifneq ($(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR)),)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/u-boot/u-boot.mk

@@ -77,7 +77,7 @@ $(U_BOOT_DIR)/.patched: $(U_BOOT_DIR)/.unpacked
 	toolchain/patch-kernel.sh $(U_BOOT_DIR) boot/u-boot \
 		u-boot-$(U_BOOT_VERSION)-\*.patch \
 		u-boot-$(U_BOOT_VERSION)-\*.patch.$(ARCH)
-ifneq ($(strip $(BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR)),"")
+ifneq ($(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR)),)
 	toolchain/patch-kernel.sh $(U_BOOT_DIR) $(BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR) u-boot-$(U_BOOT_VERSION)-\*.patch
 endif
 	touch $@