toolchain: linux-headers: Fix kernel version for ADK_UPDATE_PATCHES_GIT

When using git to apply kernel patches, the extracted kernel sources
become a git repository with HEAD not being at a tag. Therefore
$(LINUX_DIR)/scripts/setlocalversion appends '+' to the kernel version
string which ADK in turn does not expect. In consequence, 'modprobe' on
target won't find any modules. Avoid this by creating an empty
'$(LINUX_DIR)/.scmversion' file - setlocalversion recognizes it and uses
its content instead of doing its own detection.

Signed-off-by: Phil Sutter <phil@nwl.cc>

toolchain/linux-headers/Makefile

@@ -41,6 +41,7 @@ ifneq ($(ADK_DISABLE_TARGET_KERNEL_PATCHES),y)
 	$(PATCH) $(TOOLCHAIN_BUILD_DIR)/w-$(PKG_NAME)-$(PKG_VERSION)-$(PKG_RELEASE)/linux-$(KERNEL_FILE_VER) \
 		$(ADK_TOPDIR)/target/$(ADK_TARGET_ARCH)/$(ADK_TARGET_SYSTEM)/patches/$(KERNEL_FILE_VER) *.patch $(MAKE_TRACE)
 endif
+	touch $(TOOLCHAIN_BUILD_DIR)/w-$(PKG_NAME)-$(PKG_VERSION)-$(PKG_RELEASE)/linux-$(KERNEL_FILE_VER)/.scmversion
 	touch $@
 
 $(WRKBUILD)/.configured: $(TOOLCHAIN_BUILD_DIR)/w-$(PKG_NAME)-$(PKG_VERSION)-$(PKG_RELEASE)/linux-$(KERNEL_FILE_VER)/.patched