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>