systemd: wrap mtime based time with packageconfig

systemd: wrap mtime based time with packageconfig

Recently the systemd recipe grew support for setting the epoch time
at image build time. Unfortunately this is unconditional, and our
use case for the set-time-epoch PACKAGECONFIG is we have a system
requirement that our product boot with the time set to the UNIX
epoch.

Instead of trying to complicate things with either an image hook or
overriding the systemd recipe, just make setting the epoch at image
time optional, with the same PACKAGECONFIG that controls hardcoding
the systemd build time as the default epoch.

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>