Convert CONFIG_SPL_NAND_BASE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NAND_BASE
   CONFIG_SPL_NAND_IDENT

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/am3517_crane_defconfig b/configs/am3517_crane_defconfig
index 4155df5..44f6c06 100644
--- a/configs/am3517_crane_defconfig
+++ b/configs/am3517_crane_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_NAND_SIMPLE=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMDLINE_EDITING is not set
 CONFIG_SYS_PROMPT="AM3517_CRANE # "