include: Determine MODULES_DIR correctly for external/git kernels

When using external or git cloned kernels, any kind of modifications
will alter KERNELRELEASE. LEDE still tries to stage modules in
lib/modules/$(LINUX_UNAME_VERSION) and LINUX_UNAME_VERSION is based on
KERNEL_PATCHVER (indirectly) so this does not work, and we lose all
kinds of automatic modules loading.

To remedy that, just cat $(LINUX_DIR)/include/config/kernel.release
which is late enough the kernel has prepared this file, and is correctly
tracking changes done throughout the kernel.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

include/kernel.mk

@@ -51,17 +51,19 @@ else
     LINUX_UNAME_VERSION:=$(LINUX_UNAME_VERSION)-$(strip $(lastword $(subst -, ,$(LINUX_VERSION))))
   endif
 
-  MODULES_SUBDIR:=lib/modules/$(LINUX_UNAME_VERSION)
-  TARGET_MODULES_DIR := $(LINUX_TARGET_DIR)/$(MODULES_SUBDIR)
-
   LINUX_KERNEL:=$(KERNEL_BUILD_DIR)/vmlinux
 
   LINUX_SOURCE:=linux-$(LINUX_VERSION).tar.xz
   TESTING:=$(if $(findstring -rc,$(LINUX_VERSION)),/testing,)
   ifeq ($(call qstrip,$(CONFIG_EXTERNAL_KERNEL_TREE))$(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
       LINUX_SITE:=@KERNEL/linux/kernel/v$(word 1,$(subst ., ,$(KERNEL_BASE))).x$(TESTING)
+  else
+      LINUX_UNAME_VERSION:=$(strip $(shell cat $(LINUX_DIR)/include/config/kernel.release))
   endif
 
+  MODULES_SUBDIR:=lib/modules/$(LINUX_UNAME_VERSION)
+  TARGET_MODULES_DIR:=$(LINUX_TARGET_DIR)/$(MODULES_SUBDIR)
+
   ifneq ($(TARGET_BUILD),1)
     PKG_BUILD_DIR ?= $(KERNEL_BUILD_DIR)/$(PKG_NAME)$(if $(PKG_VERSION),-$(PKG_VERSION))
   endif