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/vct_premium_onenand_small_defconfig b/configs/vct_premium_onenand_small_defconfig
index 7b2d715..e7d6f91 100644
--- a/configs/vct_premium_onenand_small_defconfig
+++ b/configs/vct_premium_onenand_small_defconfig
@@ -28,4 +28,5 @@
 # CONFIG_ISO_PARTITION is not set
 CONFIG_ENV_IS_IN_ONENAND=y
 # CONFIG_NET is not set
+CONFIG_MTD_DEVICE=y
 CONFIG_SYS_NS16550=y