kernel.bbclass: Complete fix for modules symlink

The fix backported in commit aa9fc551 of oe-core does not completely fix the
issue (Yocto #4595) as intended. The modules symlink is still created in the
working directory instead of in the deploy directory. To fix this, we just need
to use an absolute path to ${DEPLOYDIR} when creating the symlink.

Signed-off-by: Paul Barker <paul.barker@commagility.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>