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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 1d07f1b..00b5e7c 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -4,7 +4,6 @@
 CONFIG_SYS_TEXT_BASE=0x80100000
 # CONFIG_SPL_GPIO_SUPPORT is not set
 CONFIG_TARGET_AM3517_EVM=y
-CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_BOOTDELAY=10
 CONFIG_VERSION_VARIABLE=y
@@ -37,6 +36,7 @@
 CONFIG_CMD_UBI=y
 CONFIG_SPL_PARTITION_UUIDS=y
 CONFIG_MMC_OMAP_HS=y
+CONFIG_NAND=y
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_MUSB_HOST=y