test: spl: Add a test for NAND

Add a SPL test for the NAND load method. We use some different functions to
do the writing from the main test since things like nand_write_skip_bad
aren't available in SPL.

We disable BBT scanning, since scan_bbt is only populated when not in SPL.
We use nand_spl_loaders.c as it seems to be common to at least a few boards
already. However, we do not use nand_spl_simple.c because it would require
us to implement cmd_ctrl.  The various nand load functions are adapted from
omap_gpmc. However, they have been modified for simplicity/correctness.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/drivers/mtd/nand/raw/Kconfig b/drivers/mtd/nand/raw/Kconfig
index 4ab5459..bb9994b 100644
--- a/drivers/mtd/nand/raw/Kconfig
+++ b/drivers/mtd/nand/raw/Kconfig
@@ -451,6 +451,8 @@
 	bool "Support for NAND in sandbox"
 	depends on SANDBOX
 	select SYS_NAND_SELF_INIT
+	select SPL_SYS_NAND_SELF_INIT
+	select SPL_NAND_INIT
 	select SYS_NAND_SOFT_ECC
 	select BCH
 	select NAND_ECC_BCH
@@ -678,7 +680,8 @@
 	depends on ARCH_SUNXI || NAND_OMAP_GPMC || NAND_LPC32XX_SLC || \
 		SPL_NAND_SIMPLE || (NAND_MXC && SPL_NAND_SUPPORT) || \
 		MVEBU_SPL_BOOT_DEVICE_NAND || \
-		(NAND_ATMEL && SPL_NAND_SUPPORT) || SPL_GENERATE_ATMEL_PMECC_HEADER
+		(NAND_ATMEL && SPL_NAND_SUPPORT) || \
+		SPL_GENERATE_ATMEL_PMECC_HEADER || NAND_SANDBOX
 	depends on !NAND_MXS && !NAND_DENALI_DT && !NAND_LPC32XX_MLC && !NAND_MT7621
 	help
 	  Number of data bytes in one page for the NAND chip on the