kbuild: Fix packaging targets with relative $(srctree)

All other users of Makefile.build set $(obj) to the name of the
subdirectory to build. Do the same for the packaging targets, otherwise
the build fails if $(srctree) is a relative directory:

    $ make O=build tar-pkg
    make[1]: Entering directory `/home/mmarek/linux-2.6/build'
      CHK     include/config/kernel.release
    ../scripts/Makefile.build:44: ../../scripts/package/Makefile: No such file or directory
    make[2]: *** No rule to make target `../../scripts/package/Makefile'.  Stop.

Fixes: 9da0763b ("kbuild: Use relative path when building in a subdir of the source tree")
Signed-off-by: Michal Marek <mmarek@suse.cz>