base-files: add warning to distfeeds.list

Add a message to the apk distfeeds.list that changes won't be saved
and that users should modify customfeeds.list instead.

Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: openwrt/openwrt#22586
Signed-off-by: Robert Marko <robimarko@gmail.com>

Author: efahl

include/feeds.mk

@@ -48,6 +48,8 @@ endef
 # 1: destination file
 define FeedSourcesAppendAPK
 ( \
+  echo '# This file is auto-generated and build-specific, any changes will be intentionally lost in sysupgrade.'; \
+  echo '# Add your custom feeds to /etc/apk/repositories.d/customfeeds.list'; \
   echo '%U/targets/%S/packages/packages.adb'; \
   $(strip $(if $(CONFIG_PER_FEED_REPO), \
 	echo '%U/packages/%A/base/packages.adb'; \