bliss: Don't enforce specifying image name on prebuilt kernel

- The goal of specifying kernel image name was to whether kernel needs
  appended dtb or not, which is not something required if the kernel is
  prebuilt

Ref:
[0]: Commit edc1cebc2f7baedf6479874db2848b45e22f4205

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>
Change-Id: I6b43756a2eac59729a67b5f80dac112f8b6bcc5b
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index fbfc50b..066ea1f 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -99,8 +99,10 @@
     KERNEL_ADDITIONAL_CONFIG_SRC := /dev/null
 endif
 
-ifeq ($(BOARD_KERNEL_IMAGE_NAME),)
-$(error BOARD_KERNEL_IMAGE_NAME not defined.)
+ifeq ($(TARGET_PREBUILT_KERNEL),)
+    ifeq ($(BOARD_KERNEL_IMAGE_NAME),)
+        $(error BOARD_KERNEL_IMAGE_NAME not defined.)
+    endif
 endif
 TARGET_PREBUILT_INT_KERNEL := $(KERNEL_OUT)/arch/$(KERNEL_ARCH)/boot/$(BOARD_KERNEL_IMAGE_NAME)