kernel.bbclass: convert base_do_unpack_append() to a task

<pre>
when&#32;externalsrc&#32;is&#32;enabled&#32;and&#32;the&#32;&#39;do_unpack&#39;&#32;task&#32;is&#32;deleted,
building&#32;kernel&#32;module&#32;fail

Signed-off-by:&#32;Steven&#32;Hung&#32;(&#27946;&#20110;&#29577;)&#32;&lt;Steven.Hung@mediatek.com&gt;
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: Steven Hung (洪于玉)

meta/classes/kernel.bbclass

@@ -130,7 +130,7 @@ inherit ${KERNEL_CLASSES}
 # the symlink.
 do_unpack[cleandirs] += " ${S} ${STAGING_KERNEL_DIR} ${B} ${STAGING_KERNEL_BUILDDIR}"
 do_clean[cleandirs] += " ${S} ${STAGING_KERNEL_DIR} ${B} ${STAGING_KERNEL_BUILDDIR}"
-base_do_unpack_append () {
+python do_symlink_kernsrc () {
     s = d.getVar("S")
     if s[-1] == '/':
         # drop trailing slash, so that os.symlink(kernsrc, s) doesn't use s as directory name and fail
@@ -147,6 +147,7 @@ base_do_unpack_append () {
             shutil.move(s, kernsrc)
             os.symlink(kernsrc, s)
 }
+addtask symlink_kernsrc before do_configure after do_unpack
 
 inherit kernel-arch deploy