kernel: Clean ${S} before unpack

Currently unpack just forces sources over the current files. This change ensures ${S} is cleaned out before sources are unpacked. This resolves issues seen when upgrading to the new kernel class changes.

Ultimately, this should probably move to base.bbclass but one step at a time, this solves an immediate problem blocking the other patches from merging.

index 3480414..9790f4d 100644
@@ -5,6 +5,8 @@ DEPENDS += "virtual/${TARGET_PREFIX}binutils virtual/${TARGET_PREFIX}gcc kmod-na
 
 S = "${STAGING_DIR_TARGET}/${KERNEL_SRC_PATH}"
 
+do_unpack[cleandirs] = "${S}"
+
 # we include gcc above, we dont need virtual/libc
 INHIBIT_DEFAULT_DEPS = "1"