arm: baltos: move CONFIG_SYS_NAND_U_BOOT_OFFS to defconfig

Also get rid of CONFIG_SYS_NAND_SPL_KERNEL_OFFS as SPL_OS_BOOT
feature won't be used.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>