include: Silence external kernel version checks

During the initial configuration phases, we have not set-up the kernel
source directory, which would lead to such messages:

cat:
/local/users/fainelli/openwrt/trunk/build_dir/target-x86_64_musl/linux-uml/linux-4.9.58/include/config/kernel.release:
No such file or directory

Just silence it, since it does not create a functional problem.

Fixes: 8e0e0e7d8b ("include: Determine MODULES_DIR correctly for external/git kernels")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

include/kernel.mk

@@ -64,7 +64,7 @@ else
   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 $(LINUX_DIR)/include/config/kernel.release))
+      LINUX_UNAME_VERSION:=$(strip $(shell cat $(LINUX_DIR)/include/config/kernel.release 2>/dev/null))
   endif
 
   MODULES_SUBDIR:=lib/modules/$(LINUX_UNAME_VERSION)