module_strip: When picking module suffix, use KERNEL_MAJOR_VERSION.

module_strip: When picking module suffix, use KERNEL_MAJOR_VERSION.
With the previously broken and unused KERNEL_OBJECT_SUFFIX, this actually
evaluted to "" so we always did a find path -name *, which is why we need
the -d test later on.  Switch to testing for KERNEL_OBJECT_SUFFIX==2.6
and else'ing for 2.4 and doing .ko or .o.  Leave KERNEL_OBJECT_SUFFIX in-tact
for non-mainline users.  This was easier than getting kernel.bbclass to
evaluate KERNEL_OBJECT_SUFFIX either on its own or re-jigging module-base and
module_strip, again for non-mainline users.