witherspoon/tacoma: Substitute variable in fan control service

The fan control service has a dependency on the max31785 hwmon service
being started where the i2c address of the max31785 is in the name of
the service file. That i2c address is different between witherspoon and
tacoma, so a systemd substitution is needed to place the correct address
based on machine into the fan control service file.

Tested:
    Built witherspoon and verified correct i2c address substituted
    Built tacoma and verified correct i2c address substituted
    Built mihawk and verified base fan control service used(no max31785)
    Built swift to verify base fan control service used(max31785)
    Verified witherspoon image on hardware

Change-Id: I5ac5ff54296326e12c08c5d7e919556e274049a8
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
2 files changed