linux: fix breakage from d4b2b032a00c9922c9efa144b014dc086a7e48f1

Unbreak qemu_xtensa_lx60_defconfig where LINUX_IMAGE_NAME !=
LINUX_TARGET_NAME.
It incorrectly overwrites LINUX_IMAGE_NAME even if it was set before,
defeating the purpose of IMAGE being different than TARGET.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

linux/linux.mk

@@ -82,7 +82,9 @@ KERNEL_DTBS = $(addsuffix .dtb,$(KERNEL_DTS_NAME))
 ifeq ($(BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM),y)
 LINUX_IMAGE_NAME = $(call qstrip,$(BR2_LINUX_KERNEL_IMAGE_NAME))
 LINUX_TARGET_NAME = $(call qstrip,$(BR2_LINUX_KERNEL_IMAGE_TARGET_NAME))
+ifeq ($(LINUX_IMAGE_NAME),)
 LINUX_IMAGE_NAME = $(LINUX_TARGET_NAME)
+endif
 else
 ifeq ($(BR2_LINUX_KERNEL_UIMAGE),y)
 LINUX_IMAGE_NAME = uImage