rcutorture: Allow Kconfig-related boot parameters to override

As it stands, the default kernel boot parameters generated from
the Kconfig fragment will override any supplied with the .boot
file that can optionally accompany a Kconfig fragment.  Rearrange
ordering to permit the specific .boot arguments to override those
generated by analyzing the Kconfig fragment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>