sunxi_nand_spl: Rename SPL_NAND_SUNXI to NAND_SUNXI

We eventually want to add full nand support, since it makes no sense
to build SPL with nand support and u-boot without, or the other way
around, a single option will suffice.

Renaming the Kconfig option now makes things easier when we add full
nand support in the future.

The "obj-$(CONFIG_NAND_SUNXI) += sunxi_nand_spl.o" is moved to an
"ifdef CONFIG_SPL_BUILD" block in the Makefile.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>