package/systemd: add checklist of upgrade caveats

Bumping the version of systemd is a little more complicated than the
usual package, and it's easy to forget some of the things that need to
be done - and this has happened several times in the past.

Add a checklist of things to worry about when updating systemd.

Note that there are many other complicated packages that also need a lot
of attention when upgrading. However, systemd is a really critical
package so it's more important that we get that one right than, say,
valgrind.

Cc: Norbert Lange <nolange79@gmail.com>
Cc: Jérémy ROSEN <jeremy.rosen@smile.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/systemd/systemd.mk

@@ -4,6 +4,21 @@
 #
 ################################################################################
 
+# When updating systemd, take care of the following:
+# - Check if the requirements have changed (see README), in particular
+#   arch and headers
+# - If yes, propagate the dependencies to BR2_INIT_SYSTEMD
+# - If the required kernel options have changed, update the Config.in
+#   help text and the list of KCONFIG_ENABLE_OPT.
+# - Check if there are new meson_options. Make sure all options are set
+#   explicitly (usually to default value).
+# - If there are new services:
+#   - create new options for them (if they really are optional);
+#   - create a new _USER if necessary;
+#   - create new directory (with _PERMISSIONS) if necessary.
+# - Diff sysusers.d with the previous version
+# - Diff factory/etc/nsswitch.conf with the previous version
+#   (details are often sprinkled around in README and manpages)
 SYSTEMD_VERSION = 250.1
 SYSTEMD_SITE = $(call github,systemd,systemd-stable,v$(SYSTEMD_VERSION))
 SYSTEMD_LICENSE = \