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/stmark2_defconfig b/configs/stmark2_defconfig
index 671c0aa..1b14aff 100644
--- a/configs/stmark2_defconfig
+++ b/configs/stmark2_defconfig
@@ -21,6 +21,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=spi-flash.0:1m(u-boot),7m(kernel),-(rootfs)"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 # CONFIG_NET is not set
+CONFIG_MTD_DEVICE=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_MTD=y
 CONFIG_SPI=y