apps: fix distclean for partial builds and residual .kconfig files

The current distclean implementation has two issues:

1. Application.mk does not remove .kconfig on distclean, so stale
   config fragments from earlier builds can pollute subsequent ones.

2. Several external-library Makefiles (lame, libshvc, libulut) run
   "make -C <subdir> distclean" without checking whether the
   subdirectory exists.  When the library was never downloaded (common
   in CI partial-build environments), distclean fails with "No such
   file or directory".

Fix:

- Application.mk: add $(call DELFILE, .kconfig) to the distclean
  target so that .kconfig is cleaned along with .built, .depend, etc.

- audioutils/lame/Makefile: guard the distclean recipe with
  "if [ -d $(DST_PATH) ]; then ...; fi" and use $(MAKE) instead
  of bare make.

- netutils/libshvc/Makefile, netutils/libulut/Makefile: same
  directory-existence guard for their distclean recipes.

- crypto/wolfssl/Makefile: change "distclean:" to "distclean::"
  (double-colon) so it does not override the distclean:: rule
  inherited from Application.mk.

Signed-off-by: hanzhijian <hanzhijian@zepp.com>

Application.mk

@@ -393,6 +393,7 @@ clean::
 	$(call CLEANAROBJS)
 	$(call CLEAN)
 distclean:: clean
+	$(call DELFILE, .kconfig)
 	$(call DELFILE, Make.dep)
 	$(call DELFILE, .depend)