spl/nand: introduce CONFIG_SPL_NAND_DRIVERS, _BASE, and _ECC.

Some small SPLs do not use nand_base.c, and a subset of those also
require a special driver.  Some SPLs need software ECC but others can't
fit it.

All existing boards that specify CONFIG_SPL_NAND_SUPPORT have these
symbols added to preserve existing behavior.

Signed-off-by: Scott Wood <scottwood@freescale.com>
--
v2: use positive logic for including bits of NAND, rather than
a MINIMAL symbol that excludes things.
diff --git a/include/configs/omap3_overo.h b/include/configs/omap3_overo.h
index 626cf7a..fd31c73 100644
--- a/include/configs/omap3_overo.h
+++ b/include/configs/omap3_overo.h
@@ -319,6 +319,9 @@
 #define CONFIG_SPL_FAT_SUPPORT
 #define CONFIG_SPL_SERIAL_SUPPORT
 #define CONFIG_SPL_NAND_SUPPORT
+#define CONFIG_SPL_NAND_BASE
+#define CONFIG_SPL_NAND_DRIVERS
+#define CONFIG_SPL_NAND_ECC
 #define CONFIG_SPL_GPIO_SUPPORT
 #define CONFIG_SPL_POWER_SUPPORT
 #define CONFIG_SPL_LDSCRIPT		"$(CPUDIR)/omap-common/u-boot-spl.lds"