tools: fix mksunxiboot build for tools-all target

Commit fed329ae ("tools: add mksunxiboot to tools-all target") added
mksunxiboot to the tools-all target, but used the CONFIG_SUNXI symbol
to enable its build. Now commit aec9a0f1 ("sunxi: Rename CONFIG_SUNXI
to CONFIG_ARCH_SUNXI"), merged before that, renamed that symbol, so that
the first patch basically gets ineffective.
Adjust the symbol name in tools/Makefile to make it build again.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>