Convert CONFIG_MTD_PARTITIONS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_MTD_PARTITIONS
   CONFIG_MTD_DEVICE

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/corvus_defconfig b/configs/corvus_defconfig
index 16999b6..b5b52c7 100644
--- a/configs/corvus_defconfig
+++ b/configs/corvus_defconfig
@@ -42,6 +42,7 @@
 # CONFIG_MMC is not set
 CONFIG_NAND=y
 CONFIG_NAND_ATMEL=y
+CONFIG_MTD_PARTITIONS=y
 CONFIG_PHYLIB=y
 CONFIG_ATMEL_USART=y
 CONFIG_USB=y