spl/nand: config symbol documentation

Document parameters used for specifying the NAND image to be loaded.

Also fix the definition of CONFIG_SPL_NAND_SIMPLE -- it's only
nand_spl_simple.c, not the entire nand directory.  The word "simple" is
there for a reason.  :-)

Signed-off-by: Scott Wood <scottwood@freescale.com>
---
v2: updated for makefile changes earlier in patchset
diff --git a/drivers/mtd/nand/Makefile b/drivers/mtd/nand/Makefile
index d8a2dfc..28e52bd 100644
--- a/drivers/mtd/nand/Makefile
+++ b/drivers/mtd/nand/Makefile
@@ -78,6 +78,10 @@
 COBJS-$(CONFIG_NAND_OMAP_GPMC) += omap_gpmc.o
 COBJS-$(CONFIG_NAND_PLAT) += nand_plat.o
 
+else  # minimal SPL drivers
+
+COBJS-$(CONFIG_NAND_FSL_ELBC) += fsl_elbc_spl.o
+
 endif # drivers
 endif # nand