classes/nativesdk: set SDK_OLDEST_KERNEL appropriately

classes/nativesdk: set SDK_OLDEST_KERNEL appropriately

SDK_OLDEST_KERNEL currently only controls the check on SDK installation,
however as with OLDEST_KERNEL it should be controlling the OLDEST_KERNEL
value for building glibc used in the SDK. Thus, set it in
nativesdk.bbclass. This means we need to move the default to
bitbake.conf so that it can be seen in both places.

Also set a more reasonable default for SDK_OLDEST_KERNEL for x86/x86-64 as
glibc 2.24 still supports back to 2.6.32 there and there are still
people wanting to build SDKs that will install on older distros (e.g.
CentOS 6). However it's not possible to set this with overrides since
there aren't any for the SDK_ARCH, however we can instead set the variable
from conf files in conf/machine-sdk especially as there is now a soft
default for SDKMACHINE.

Fixes [YOCTO #10561].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>