starting phosphor-hwmon with 4.13

[Matt Spinler]

Patrick, others,

With the 4.13 kernel, the OCCs moved out of the device tree.  As such, 
the udev rule that is currently used to start phosphor-hwmon that starts 
the service with the OF_FULLNAME udev attribute will no longer work.

In addition, the systemd capability of using SYSTEMD_WANTS in a udev 
rule to start a templated service is something that upstream rejected 
(maybe just needs further work?), and we are just carrying as a patch, 
so if we can get rid of it that would be a bonus.

So this is what I'm proposing:

Change the udev rule to start and stop hwmon without systemd's help:

SUBSYSTEM=="hwmon", ACTION=="add", RUN="/usr/bin/start_hwmon.sh start 
%p $env{OF_FULLNAME}"
SUBSYSTEM=="hwmon", ACTION=="remove", RUN="/usr/bin/start_hwmon.sh stop 
%p $env{OF_FULLNAME}"


where start_hwmon.sh does a systemctl start of 
xyz.openbmc_project.Hwmon at .service, passing it OF_FULLNAME if it's 
there, otherwise a shortened version of the devpath (takes out the hwmon 
instance, since that is variable so we can't use it to look up env files).

Then I would just need make a minor modification to phosphor-hwmon to 
handle the devpath, and move the OCC config files into a new location in 
/etc/default/obmc/hwmon, and nothing else should really need to change.

Please let me know if anyone has any better ideas, or other comments.