Convert CONFIG_NAND_ATMEL to Kconfig

This converts the following to Kconfig:
   CONFIG_NAND_ATMEL

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/corvus_defconfig b/configs/corvus_defconfig
index 0f91f6e..16999b6 100644
--- a/configs/corvus_defconfig
+++ b/configs/corvus_defconfig
@@ -40,6 +40,8 @@
 CONFIG_CLK_AT91=y
 CONFIG_DFU_NAND=y
 # CONFIG_MMC is not set
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_PHYLIB=y
 CONFIG_ATMEL_USART=y
 CONFIG_USB=y