Makefile: Update mtime of $(TARGET_DIR)/usr in target-finalize

Makefile: Update mtime of $(TARGET_DIR)/usr in target-finalize

The systemd ConditionNeedsUpdate option is useful when offline updates
of the vendor operating system resources in /usr require updating of
/etc or /var on the next following boot.

Two examples of services making use of this option are
systemd-hwdb-update.service and systemd-sysusers.service.

ConditionNeedsUpdate=/etc will be true if the mtime of /etc/.updated
is older than the mtime of /usr.  After services conditional on
ConditionNeedsUpdate have run, systemd-update-done.service will
synch the mtime of /usr to /etc/.updated so that the condition will
be false on subsequent boots.

For systems with writable /usr partitions where updates are done to
the running system, the update program will touch /usr as a final step.
But with Buildroot, where updates are often done by dumping a new
image onto the device, and where /usr is on a filesystem mounted
read-only, touching /usr as part of the update process is not practical.
Instead, it should be done a build time.

For testers, please note that systemd-update-done in v234 added a
regression where the mtime of /etc/.updated is set to the current time
instead of the mtime or /usr.  This will be fixed in v239.

For more details, see:
http://0pointer.de/public/systemd-man/systemd.unit.html
http://0pointer.de/public/systemd-man/systemd-update-done.service.html

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>