fix for MACHINE_KERNEL_PR stuff

fix for MACHINE_KERNEL_PR stuff

This patch unbreaks current behaviour which was introduced by
MACHINE_KERNEL_PR variable.

As most of target machines do not use it they have PR with broken value
(set to "r0" instead of value in recipe). I took other way which makes
both types of users happy -- those with MACHINE_KERNEL_PR in use and
those without it.

By default we set M_K_PR to empty string instead of "r0" - this allows
to check is it set at all or not. If it is set then we set PR to this value.
Otherwise we ignore existance of that variable and use PR from recipe.

Signed-off-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>