Convert CONFIG_NAND to Kconfig

This converts the following to Kconfig:
   CONFIG_NAND

Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Sync up a few more, add imply's]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/P2020RDB-PC_NAND_defconfig b/configs/P2020RDB-PC_NAND_defconfig
index 91d338a..9d94662 100644
--- a/configs/P2020RDB-PC_NAND_defconfig
+++ b/configs/P2020RDB-PC_NAND_defconfig
@@ -11,7 +11,6 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_BOOTDELAY=10
 CONFIG_BOARD_EARLY_INIT_F=y
@@ -34,6 +33,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTD_NOR_FLASH=y
+CONFIG_NAND=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_PHYLIB=y