build: Allow kernel modules to set build ID debug symbol

This change adds support for specifying a build ID for kernel modules.
This is done by setting PKG_BUILD_ID to a hexadecimal string, which will
then be passed to the kernel linker. In addition, when this flag is set,
the build ID debug symbol (.note.gnu.build-id) will not be stripped from
the kernel module. This symbol is exported in sysfs by the kernel (if
the kernel is compiled with CONFIG_KALLSYMS) and so can be used to
uniquely identify a version of a kernel module in a running kernel. This
is useful for keeping track of different versions of a module when doing
experiments and development.

Modules that specify the build ID will be ~100 bytes larger (depending
on the length of the build ID specified). There is no size difference
for kernel modules that do not set this variable.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

SVN-Revision: 47290

include/kernel-defaults.mk

@@ -11,7 +11,8 @@ KERNEL_MAKEOPTS := -C $(LINUX_DIR) \
 	ARCH="$(LINUX_KARCH)" \
 	KBUILD_HAVE_NLS=no \
 	CONFIG_SHELL="$(BASH)" \
-	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='')
+	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \
+	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID))
 
 ifdef CONFIG_STRIP_KERNEL_EXPORTS
   KERNEL_MAKEOPTS += \