Cleanup CONFIG_VERSION_VARIABLE migration

CONFIG_VERSION_VARIABLE had already been mostly migrated to Kconfig,
but two headers still referenced it. Also set defaults so that
most of the defconfigs inherit the default rather than needing it
to be set explicitly.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>