nginx: fix nginx.old clean-up

The bin path for nginx is configured for /usr/sbin, so deleting
the nginx.old back-up from /usr/bin never works.  Fix path, and
also use "$(RM)" instead of "-$(RM)", as "rm -f" never fails.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/nginx/nginx.mk

@@ -222,7 +222,7 @@ endef
 
 define NGINX_INSTALL_TARGET_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D) DESTDIR=$(TARGET_DIR) install
-	-$(RM) $(TARGET_DIR)/usr/bin/nginx.old
+	$(RM) $(TARGET_DIR)/usr/sbin/nginx.old
 	$(INSTALL) -D -m 0664 package/nginx/nginx.logrotate \
 		$(TARGET_DIR)/etc/logrotate.d/nginx
 endef