kernel.bbclass: run do_symlink_kernsrc before do_patch

There's a race between do_symlink_kernsrc and do_populate_lic, since
the latter is ordered "after do_patch"; so the two may run in
parallel. In some cases, that actually causes do_populate_lic to fail
if it happens to look for a license file somewhere under ${S} in the
short window after shutil.move and before the symlink has been
created.

Fix that by simply ordering symlink_kernsrc before do_patch. Any task
that pokes around in ${S} looking for files should be ordered after
do_patch, so this should also fix similar latent races with other ad
hoc tasks.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>