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/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index 34a6975..54f8bcd 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -5,7 +5,6 @@
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y
@@ -32,6 +31,7 @@
 CONFIG_DFU_NAND=y
 CONFIG_DFU_RAM=y
 CONFIG_MMC_OMAP_HS=y
+CONFIG_NAND=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_PHYLIB=y