kernel: Pass absolute path for KCONFIG_ALLCONFIG

It fails to read relative one, at least on QCOM 4.19 kernel base.

Change-Id: Iacbcadfabfcebb80a7f5934825123fdfa913c63c

build/tasks/kernel.mk

@@ -288,7 +288,7 @@ $(KERNEL_CONFIG): $(KERNEL_DEFCONFIG_SRC) $(KERNEL_ADDITIONAL_CONFIG_OUT)
 	$(hide) if [ ! -z "$(KERNEL_ADDITIONAL_CONFIG)" ]; then \
 			echo "Using additional config '$(KERNEL_ADDITIONAL_CONFIG)'"; \
 			$(KERNEL_SRC)/scripts/kconfig/merge_config.sh -m -O $(KERNEL_OUT) $(KERNEL_OUT)/.config $(KERNEL_SRC)/arch/$(KERNEL_ARCH)/configs/$(KERNEL_ADDITIONAL_CONFIG); \
-			$(call make-kernel-target,KCONFIG_ALLCONFIG=$(KERNEL_OUT)/.config alldefconfig); \
+			$(call make-kernel-target,KCONFIG_ALLCONFIG=$(KERNEL_BUILD_OUT_PREFIX)$(KERNEL_OUT)/.config alldefconfig); \
 		fi
 
 $(TARGET_PREBUILT_INT_KERNEL): $(KERNEL_CONFIG) $(DEPMOD) $(DTC)