For some architectures is does not make sense to maintain a single default
configuration - arm is a good example here.
KBUILD_DEFCONFIG is introduced allowing relevant architectures to point out
a configuration kept in arch/$(ARCH)/configs as the configuration
to be used when executing 'make defconfig'.
This patch selects versatile_defconfig as the default configuration for arm.

The reason to keep defconfig is that is has proved valuable in many testing
scenarios when one are doing a cross architecture build of the kernel to
do a simple compile-time check of some changes.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>