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_defconfig b/configs/am335x_evm_defconfig
index 6dcb58f..e05f50e 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -6,7 +6,6 @@
 CONFIG_DEFAULT_DEVICE_TREE="am335x-evm"
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SPL_LOAD_FIT=y
-CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y
@@ -36,6 +35,7 @@
 CONFIG_MISC=y
 CONFIG_DM_MMC=y
 CONFIG_MMC_OMAP_HS=y
+CONFIG_NAND=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_PHYLIB=y