image: fix CONFIG_EXTERNAL_CPIO handling

CONFIG_EXTERNAL_CPIO is a string variable, hence testing for 'y'
doesn't make much sense here.

Fixes: 330bd380e8 ("image: allow building FIT and uImage with ramdisk")
Reported-by: Huangbin Zhan <[email protected]>
Signed-off-by: Daniel Golle <[email protected]>

include/kernel-defaults.mk

@@ -171,7 +171,7 @@ define Kernel/CompileImage/Initramfs
 	$(if $(SOURCE_DATE_EPOCH),touch -hcd "@$(SOURCE_DATE_EPOCH)" $(TARGET_DIR)/init)
 	rm -rf $(KERNEL_BUILD_DIR)/linux-$(LINUX_VERSION)/usr/initramfs_data.cpio*
 ifeq ($(CONFIG_TARGET_ROOTFS_INITRAMFS_SEPARATE),y)
-ifeq ($(CONFIG_EXTERNAL_CPIO),y)
+ifneq ($(qstrip $(CONFIG_EXTERNAL_CPIO)),)
 	$(CP) $(CONFIG_EXTERNAL_CPIO) $(KERNEL_BUILD_DIR)/initrd.cpio
 else
 	( cd $(TARGET_DIR); find . | $(STAGING_DIR_HOST)/bin/cpio -o -H newc -R 0:0 > $(KERNEL_BUILD_DIR)/initrd.cpio )