arm: mvebu: Fix default CONFIG_BUILD_TARGET

u-boot-with-spl.kwb is built only for SPL enabled 32-bit armada boards.
u-boot.kwb is built for 32-bit armada and kirkwood boards but only for
non-SPL targets.

So replace CONFIG_ARCH_MVEBU by CONFIG_ARMADA_32BIT (it implies
CONFIG_ARCH_MVEBU) for u-boot-with-spl.kwb.

And add additional CONFIG_ARMADA_32BIT && !CONFIG_SPL for u-boot.kwb.

Signed-off-by: Pali Rohár <pali@kernel.org>