core: remove .br-external on distclean

distclean is supposed to return the current directory, whether in-tree
or out-of-tree, into pristine conditions, which means we should also
forget about any br2-external tree on distclean.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -844,7 +844,7 @@ ifeq ($(O),output)
 	rm -rf $(O)
 endif
 	rm -rf $(BR2_CONFIG) $(CONFIG_DIR)/.config.old $(CONFIG_DIR)/..config.tmp \
-		$(CONFIG_DIR)/.auto.deps
+		$(CONFIG_DIR)/.auto.deps $(BR2_EXTERNAL_FILE)
 
 help:
 	@echo 'Cleaning:'