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/README b/README
index 44cd65a..af8ab69 100644
--- a/README
+++ b/README
@@ -2716,6 +2716,16 @@
 		CONFIG_SPL_FAT_LOAD_PAYLOAD_NAME
 		Filename to read to load U-Boot when reading from FAT
 
+		CONFIG_SPL_NAND_BASE
+		Include nand_base.c in the SPL.  Requires
+		CONFIG_SPL_NAND_DRIVERS.
+
+		CONFIG_SPL_NAND_DRIVERS
+		SPL uses normal NAND drivers, not minimal drivers.
+
+		CONFIG_SPL_NAND_ECC
+		Include standard software ECC in the SPL
+
 		CONFIG_SPL_NAND_SIMPLE
 		Support for drivers/mtd/nand/libnand.o in SPL binary