payloads/external/leanefi/Makefile: Fix clean target

Just follow the examples of other payloads and simply remove the build
directory of said payload.

Change-Id: Idf2a8f3b9ecbb300514d2d1deede76785fd402b7
Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82897
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
diff --git a/payloads/external/leanefi/Makefile b/payloads/external/leanefi/Makefile
index 4edec32..62d2cd6 100644
--- a/payloads/external/leanefi/Makefile
+++ b/payloads/external/leanefi/Makefile
@@ -17,6 +17,6 @@
 	rm -rf leanefi
 
 clean:
-	$(MAKE) -C leanefi clean
+	rm -rf leanefi/build
 
 .PHONY: build clean distclean