torture: Save a few lines by using config_override_param initially

This commit saves a few lines of code by also using the bash
config_override_param() to set the initial list of Kconfig options from
the CFcommon file.  While in the area, it makes this function capable of
update-in-place on the file containing the cumulative Kconfig options,
thus avoiding annoying changes when adding another source of options.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>