kernel: copy defconfig to {B} vs {S}

For kernel's that use a split source/object build the copy
of defconfig to {S} in the base kernel class is problematic.
The previous solution for this was to override the do_configure
of the base kernel class in a subclass. While this is still
a viable/valid option, it does mean that changes to the base
do_configure will be missed.

The solution to this is to copy a defconfig to {B} which is
typically the same as {S}, so most kernel recipes won't see or
care about this change.

With this change in place, linux-yocto.bbclass can drop its
override of do_configure.

Tested with linux-yocto and oe linux recipes.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>