[source] include: Silence external kernel version checks

[LEDE Commits]

florian pushed a commit to source.git, branch master:
https://git.lede-project.org/474391573bbd9720a58b6fcd725896f85113e4ec

commit 474391573bbd9720a58b6fcd725896f85113e4ec
Author: Florian Fainelli <f.fainelli at gmail.com>
AuthorDate: Tue Oct 24 16:59:15 2017 -0700

    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: 8e0e0e7d8bfb ("include: Determine MODULES_DIR correctly for external/git kernels")
    Signed-off-by: Florian Fainelli <f.fainelli at gmail.com>
---
 include/kernel.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/kernel.mk b/include/kernel.mk
index a84c755..75797cf 100644
--- a/include/kernel.mk
+++ b/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)