[backend] Make the source service default to the same path as the rcobsservice script (which creates and chowns the dir if needed for default setup) but don't force the use of that dir in the rcobsservice script to allow overriding when the default /var/tmp/ is not desired. Otherwise the setting in BSConfig.pm is essentially ignored.

Signed-off-by: David Greaves <david.greaves@jolla.com>