build: Fix not altering KERNELRELEASE for external kernel

When an external kernel tree is used the version should not get
modified by the LEDE build scripts. This was added by Florian some time
ago.
The commit 0aed054bec ("build: add KERNEL_MAKE and
KERNEL_MAKE_FLAGS variables and move to kernel.mk") breaks this feature
introduced in b6746a6ffb ("include: Do not alter KERNELRELEASE for
external/git kernels").

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

include/kernel.mk

@@ -108,11 +108,10 @@ KERNEL_MAKE_FLAGS := \
 	CONFIG_SHELL="$(BASH)" \
 	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \
 	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID)) \
-	KERNELRELEASE=$(LINUX_VERSION) \
 	cmd_syscalls=
 
 ifeq ($(call qstrip,$(CONFIG_EXTERNAL_KERNEL_TREE))$(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
-  KERNEL_MAKEOPTS += \
+  KERNEL_MAKE_FLAGS += \
 	KERNELRELEASE=$(LINUX_VERSION)
 endif