kbuild: create the source symlink earlier in the objdir

It's useful to already have the source symlink in a
objdir when one just runs make *config. Then one
can do

mkdir obj-allyes
cd obj-allyes
make -C ../sourcedir O=$(pwd) allyesconfig
./source/scripts/config --disable debug_info
make CC=icecc -j18

without having to interrupt the make first just to
get the source symlink.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
[sam: deleted the other source symlink statement]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>