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/am335x_hs_evm_uart_defconfig b/configs/am335x_hs_evm_uart_defconfig
index f52cc96..339a995 100644
--- a/configs/am335x_hs_evm_uart_defconfig
+++ b/configs/am335x_hs_evm_uart_defconfig
@@ -27,6 +27,7 @@
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_BASE=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y
 # CONFIG_CMD_SETEXPR is not set