util/amdfwtool,post: add missing distclean target

Without this target some spurious errors occurred when running make
distclean at the top level of coreboot.

Change-Id: I3d3061b386fc5b4a043cfc7ff8fd3c0da33c0e83
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49227
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/amdfwtool/Makefile

@@ -16,3 +16,5 @@ $(TARGET): $(OBJ)
 
 clean:
 	@rm -f $(TARGET) $(OBJ)
+
+distclean: clean

util/post/Makefile

@@ -2,3 +2,5 @@ all:
 	$(CC) post.c -o post
 clean:
 	rm -f post
+
+distclean: clean