meta-phosphor: disable 'uninative' for RHEL6 support

The uninative stub from Yocto-2.2 includes a glibc that
no longer has legacy kernel support for the kernel in
RHEL6.  We need to disable uninative and set OLDEST_KERNEL
to enable backwards compatibility.

See openbmc/openbmc#1407 to track removal of this.

Change-Id: I6352514016d803b7b77d483425b3f080eef896c0
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>