Import fix from LTSI: 3.4+ RT kernels use CONFIG_PREEMPT_RT_FULL

Initial LTSI commit:

From: Paul Gortmaker <paul.gortmaker@windriver.com>
> fix reference to obsolete RT Kconfig variable.
>
> The preempt-rt patches no longer use CONFIG_PREEMPT_RT in
> the 3.4 (and newer) versions.  So even though LTSI doesn't
> include RT, having this define present can lead to an easy
> to overlook bug for anyone who does try to layer RT onto
> the LTSI baseline.
>
> Update it to use the currently used define name by RT.
>
> Reported-by: Jim Somerville <Jim.Somerville@windriver.com>
> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

Merged with kernel version checks for >= 3.4 to support both old and
newer kernels.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>