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

rules.mk

@@ -273,8 +273,9 @@ else
       STRIP:=$(STAGING_DIR_HOST)/bin/sstrip
     endif
   endif
-  RSTRIP:= \
+  RSTRIP= \
     export CROSS="$(TARGET_CROSS)" \
+		$(if $(PKG_BUILD_ID),KEEP_BUILD_ID=1) \
 		$(if $(CONFIG_KERNEL_KALLSYMS),NO_RENAME=1) \
 		$(if $(CONFIG_KERNEL_PROFILING),KEEP_SYMBOLS=1); \
     NM="$(TARGET_CROSS)nm" \